frog embryos
DESCRIPTION
A guide book containing discussions about the stages of the embryo of a frog.TRANSCRIPT
6@zFreg Embrye,s
A. INTRODUCTION
Amphibian eggs are relatively large and can be read-ily obtained. Many outstanding experimental embry-ologists (including Hans Spemann, Ross Harrison, andtheir several students) took advantage of these factswhen they began to experiment on developing verte-brate embryos by removing parts, adding parts, and re-combining parts by microsurgery. Because so much ofclassical experimental embryology involved experimentson early amphibian embryos, it is essential that you havean understanding of the structure of these embryos andhow this sffucture originated in order to appreciate ex-perimental analyses. The study of early amphibian de-velopment, when compared to that of other organismscovered in this laboratory guide and atlas' will alsodemonstrate the basic similarities in developmentalevents and processes among different vertebrates suchas amphibians, birds, and mammals, and between thesevertebrates and invertebrates such as sea urchins. Forhands-on studies using frog embryos, see Chapter 6(Exercises 2.1-2.4).
B. HOW TO USE SERIAL SECTIONS
Laboratory work in introductory courses in develop-mental biology and embryology usually includes studiesof developmental anatomy. Suppose you obtained somepreserved frog embryos at the stage illustrated by Fig.2.La andwanted to study their anatomy. With a micro-scope, you could identify a few poorly defined externalfeatures as well as the body axes (cranial-caudal, dorsal-t'entral, andright-left). To study internal features in de-tail,you could slice (section) the entire embryo into thinsections of a given thickness; sections cut perpendicu-larly to the cranial-caudal axis of the embryo are calledtansverse (cross) sections. A slide or a collection ofslides containing every transverse section from the firstone (the most anterior one) to the last one (the mostpos-rcrior one) is called a set of serial transverse sections. T}:remost anterior section of this set (#1) is mounted at the
upper left-hand corner of the slide, and successive sec-tions are mounted in the following way (the numbersshown are for illustrative purposes only;the actual num-bers of sections in each row, and the number of rows perslide, vary):
r 2 3 4 5 6 7 8 9101.1. 12 13 14 15 16 17 18 19 20
21. 22 23 24 25 26 27 28 29 30
If there are too many sections to mount on one slide, themore posterior ones are mounted on slide #2 of the se-ries in the following way:
31 32 33 34 35 36 37 38 39 4041 42 43 44 45 46 47 48 49 50
and so forth.
One other type of serial section will be encountered ex-tensively in your laboratory studies: the sagittal section.A set of serial sagittal sections cuts parallel to the longaxis of the embryo. A midsaginal section is cut exactlydown the midline of the embryo, whereas a parasagittalsection is cut either to the right or left of the midline.Thus sagittal sections pass through the dorsal-ventral ex-tent of the embryo, either to the pight or left of the mid-line or on the midline.
Fig. 2.1b illustrates four representative transv ers e sec-tions. Fig.2.l-c illustrates these same sections after theywere transferred to a glass slide and mounted from leftto right in the order in which they were cut. Exactly howmuch of the anatomy of the embryo can one expect tosee in any one representative section? Suppose that yourset of serial sections contained a total of 100 sections.The sixth section of the set might cut through the levelof the developing eyes; in this section you could deter-mine the relationship of the eyes to other structures(Figs. 2.1b,2.1c). You might then examine a more pos-terior section, such as #1-5 through the heart,or still moreposterior sections (#50, #80). Unfortunately, the studyof individual sections provides only a two-dimensionalpicture of the embryo. To understand the anatomy of
26 Chapter 2
the embryo inthree dimensions you must visualize eachsection as again part of the whole embryo. For exam-ple, the notochord can be identified in sections #15, #50,and #80. By connecting the section of the notochordat the level of section #15 with the section of the noto-chord at the next level (section #50) and those at suc-cessive levels, you get an accurate picture of thecraniocaudal extent of the notochord, as well as its re-lationship to other structures. In the same way, youcan get an accurate picture of the craniocaudal extentof the neural tube (brain and spinal cord) and digestivetube, as well as their relationship to each other and toother structures. The most dfficult task facing the be-ginning student is to learn to visualize relationships ofparts of an embryo to one another in three dimensions.We have attempted to help you with this difficult task
by providing three types of visual aids in this laborato-ry manual: (1) line drawings (called "Figs."); (2) sec-tion "orientators," placed on most photo legends (theseorientators show the exact levels where embryos weresectioned, sliced, or cryofractured); and (3) scanningelectron micrographs, which portray a more three-di-mensional image than do flat, two-dimensional serialsections.
Methods have been included in Chapter 6 (AdvancedHands-on Studies) to help you understand how embryosare prepared for light microscopy, scanning electron mi-croscopy, immunocytochemistry, and in situ hybridiza-tion, as well as to help you understand why differentprocessing procedures result in different types of im-ages.
cranial
section numbers
cranial
caudal
caudal
anterior
section numbers
posterior
HEART DIGESTIVETUBE
ventral
EYE BRAIN (NEURALTUBE) SPINALCORD
Fig. 2.1. Drawings illustrating the relationships between a preserved 4-mm frog embryo viewed from the leftside (Fig. Z.Ia) and four representative transverse sections through this embryo (Figs. 2.1b,2.1c). Section num-bers are for example only,
Frog Embryos 27
C. OOGENESIS ANDFERTILIZATION
Oryenesis (development of the ovum) begins in thd:i.1.-:;d or-aries of the mature female frog. During therricoJins season (which begins in the spring), each ovary:r- csxts of a sac containing a cluster of spherical struc-*;.s r-alled follicles (F19.2.2). Each follicle consists ofr -::ge central cell containing a lot of yolk in its cyto-:L:s:r. the primary oocyte, surrounded by a layer ofn,;h >maller, flattened cells called foilicle cells. The:r::rn ooclte contains a large nucleus, the germinal,nqritCe. The vitelline membrane lies between the folli-: e :,eLis and the plasmalemma of the primary oocyte.r- ::;r sheath ofconnective tissue,the thecafolliculi ex-mcrnra- forms the surface layer of the ovary. Anotherrir::ih. the theca folliculi interna, partially surroundsr r':i lollicle but is lacking in the region where the fol-llj:: J{,rntacts the theca folliculi externa; at this regione,muftrtion (the rupture of the follicle and release of its:, - ri::.ined oocyte) occurs.:,t:: qlr-3ry also contains cells called oogonia. These:r*. '.mdergo rapid mitotic divisions, increasing in num-T'i'i -{ller the breeding season is completed (that is, in.[T. : iiumn). a few thousand oogonia within each ovary,:5- --he ability to divide mitotically. Each enlarges,,Lir.:--r' as a primary ooclte and becomes surrounded"r:,r l. lmgle layer of follicle cells, forming an ovarianfrultrhde. s'hich slowly enlarges due to the accumulationt' "'- '-r. These primary ooqtes enter the prophase stager: :-:; ftrst meiotic division but remain there until the
yolk-laden cytopl ot pnmary oocyte
germinal vesicle offollicle cellsprrmary oocyte vitelline
membrane
theca folliculi externa\--/-----J
site of ovulation
theca folliculi interna
following spring.
Fully grown primary oocytes undergo ovulation in re-sponse to hormones secreted by the anterior pituitarygland (adenohypophysis). Each oocyte is slowlysqueezed through the follicular wallat the region wherethe theca folliculi interna is lacking (Fig.2.2) andentersthe body cavity (coelom) of the female. Many oocytes(2,000-20,000, depending upon the species) are or,ulat-ed by a single female each breeding season. primaryoocytes complete the first meiotic division during ovu_lation, with each forming a first polar body and sec-ondary oocy'te. Both of these structures are containedwithin the vitelline membrane formed earlier, while theprimary oocyte was in the ovary. Cilia on the lining ofthe coelom beat toward the ostium of the oviduct andpropel the secondary oocytes into this opening. The sec-ond meiotic division is initiated by each secondaryoocyte at about the time that it enters the oviduct, butit then arrests in the metaphase stage. As the secondaryoocytes pass through the oviduct, a multilayered, gelati_nous egg capsule is secreted outside of the vitellinemembrane by the cells lining the oviduct.
'Fertilization occurs externally as the secondary oocytes
are spawned (shed) by the female into the water. Thesecond meiotic division is completed as a sperm con_tacts and penetrates each secondary oocyte, resulting inthe formation of a second polar hody and a matureovum containing the female pronucleus. The nucleus'of the penetrating sperm enlarges within the ovum asthe male pronucleus, and the male and female pronu_clei unite to complete the process of fertilization.
Eimi .} -) Schematic drawing of a section through three ovarian follicles of a mature female frog.
,n:!! '- '28 Chapter 2
D. FORMATION OF THE GRAYCRES.CENT
The outer portion of the egg (the cortex) contains a dis_tinct pattern of pigmentation at the time of ovulation(Fig.2.3a). About two-thirds of the cortex is heavily pig_mented;the remainder contains almost no pigment. Theuppermost part of the pigmented portion is the animalpole. This pole corresponds to the cranial end of the fu_ture embryo. The half of the egg that contains the an_imal pole is the animal hemisphere. The vegetal polelies directly opposite the animal pole. This pole corre_sponds tothe caudalend of the future embryo. The halfof the egg that contains the vegetal pole is the vegetal
b
gray crescent
hemisphere.
Following contact by and entrance of the sperm, the pig-mented cortex sffis relative to the less pigmented deep_er portion of the egg, toward and past the site of spermentrance and away from the side of the egg opposite thesperm entrance point (Fig.2.3b). This reduces the pig_mentation of a crescent-shaped area opposite the pointof sperm entrance. This crescent-shaped area betweenthe heavily pigmented cortex above ind the essential-ly nonpigmented cortex below constitutes the gray cres,cent (Figs. 2.3b,2.3c). A plane passing rhrough theanimal and vegetal poles and through the center of thegray crescent corresponds to the midsagittal plane of thefuture embryo (Fig. 2.3c).
Ap point of sperm entrance 4p midsagi t ta l p lane of
direction of shift offuture embryo
pigmented corte
gray crescent
gray crescent
unfertilized egg(secondary oocyte stage)
VPfertilized egg
(right side view)
VP4-cell stage
(dorsal-right side view)
VPfertilized egg(dorsal view)
gray crescent
dorsalblastoporal
ltp I
4 micromeres
4 macromeres
yolk plug
VP2-cell stage
(dorsal-right side view)
VPblastula stage
(dorsal-right side view)
VP blastdorsal lip gastrula stage
(dorsal view)
VP8-cell stage
(dorsal-right side view)
yolk piug gastrula stage(caudal view)
Fig. 2.3. Drawings of early developmental stages of the frog. Ap, animal pole;D, aorrd sa"JJ"t s0"3,right side;V, ventral side;Vp, vegetal pole.
E- CTEAVACE ANDMLASTUTATION
Frog Embryos 29
(2) it is four or five cells thick, with smaller cells andfewer cell layers at the animal pole and with a pro_gressive increase in cell size and number of cell layerstoward the vegetal pole;and (3) the blastomeres con_tain very little yolk.
Examine the vegetal hemisphere. Its characteristics areexactly the opposite: (1) a pigmented cortex, if presentat all, is much less evident than in the animal hemi_sphere; (Z) the blastomeres are very large and few innumber,indicative of less frequent cleavage;and (3) theblastomeres are packed with yolk.Try to identify the gray crescent. In your sections it usu-ally lies either to the left or right side of the blastocoeland also slightly ventral to it. It has the following char_acteristics (compare sides indicated by letters D andVin Photo 2.1): (1) the pigmented cortex is thinner inthe gray crescent than on the opposite side;and (2) theblastocoel lies nearer the surface on the grav crescentside than on the opposite side (thar is, th"e wall of theblastula is thinner on the gray crescent side than onthe opposite side).
The blastula consists of a mosaic of cellular areas, eachof which will normally produce a certain structure dur_ing subsequent development. In other words, each areaof cells has a certain prospective fate that will be real_ized during normal development. In blastulae of somechordates (Urochordata or tunicates) the outlines ofthese cellular areas can be determined directly becausethe cytoplasm of cells within certain areas is colored dif_ferently. But in most cases it is necessary to determinethe prospective fate of each cellular area indirectlv bvmarking experiments. Vital dyes0have been used mostfrequently for this purpose in amphibians. Several areasof the blastula are stained with a vital dye, and the struc_ture or structures that are formed from each stainedarea are observed (see Chapter 6, Exercise 2.2\.Another technique has been used more recently. A cellmarker (for example, the enzyme horseradish peroxi_dase, which can be demonstrated histochemicallv bv in_cubating tissue containing the enzyme wittr
-ttre
appropriate substrate; or fluorescein- or rhodamine-labeled dextran, which can be demonstrated with a flu_orescence microscope after illumination with the properwavelength of light) is injected into a single celf orgroups of cells at the blastula stage. As injected cellscleave, the marker is passed to their descendants. Aprospective fate map is constructed with the aid of theinformation gained by these techniques. The amphib_ian fate map should be carefully compared with the onesfor the chick (Fig. 3.7), mouse (Fig.4.2),and sea urchin(Fig. 1.1). A prospective fate map indicates the locationof specific groups of cells prior to the onset of gastru-lation. These groups of cells are shifted in an olderlyway into appropriate positions during gastrulation,which will enable them to cooperate and interact in for_mation of tissues and organs. The blastocoel appears to
'iltii.t 'i '
lt{.'t:
ttil$unr,r€te consists of a series of rapid mitotic divisionsltlriti: :-suh in blastulation (that is, the formation of a
IttlMr- rvhich consists of a group of cells called blas-surrounding a cavity, the blastocoel). Cleavage
llimryrs pass through the entire egg, so cleavage is clai_n,nil*: .s total (holoblastic). lloweve4 furrows passtmr'trr; rhe vegetal hemisphere much more slowly thanrt[Tn:r,:.h the animal hemisphere, presumably becauserITi. :t:t:rer contains far more yolk than the latter. A1lr ln(r :fotetable of the first three cleavages for the frog,fl*mnu ::rrens, is as follows (Hgs.2.3d-2.3f): (1) first cleav_.'$t :eridional (that is, passes through both the ani_urtu. 'l.i r-egetal poles), usually bisecting the gray;n*s::nr (rhat is, passes through the midsagittal planenr .:; :urure embryo), 2.5 hours postfertilization; (2)rrcr;-Ed cleavage: meridional, at right angles to the first,I 5 -':ur: postfertilization; and (3) third cleavage:hor_nr;ni;* displaced toward the animal pole due to the yolkr:mr:ir of the vegetal hemisphere, producing four small_rr r-rrnai hemisphere cells, the micromeres, and four .lii{'r.: \ egetal hemisphere cells, the macromeres,4 hoursprur:r-nilization. The egg reaches the blastula stage nearllrs ::d of cleavage (approximately 16 hours postfer_urLi,ru.:itrn I (Fig. 2.3 g). Numerous small blastomeres oc_;ur", rhe animal hemisphere, whereas the vegetaln*rr:sahere consists of a lesser number of larger blas_ilme:c! The blastula is only slightly larger at the endrr :i€.arage than is the newly fertilized egg.;:r;:rne models, or preferably living eggs (see Chaptern. =u;rcise 2.1), showing early cleavage stages. Note thetlilir$irtr-rn of early cleavage furrows (that is, whethermrn:lt-rnal or horizontal), the difference in size of theTm{":r tnneres and macromeres, and the difference in pig_meL:rrlion of animal and vegetal hemisphere cells. Tiy:iLr i[€ntifi' the gray creseent; the region of the gray cres_i$rf,: :hat is broadest will become the dorsal surface of:lLre ;unbn'o. and the opposite side of the egg will be_r:m; rhe ventral surface.:Tal-ne a section of the frog blastula that closely re-ln*r:ies Photo 2.1. Identify the main cavity, the blas-mcffil- \ore that it is displaced toward one side of therl;-N$mLla. the animal pole; thus, it is contained within1lLT.c nrrimel hemisphere. The blastocoel is filled with a1[n.ur,: ;frs1 may have coagulated during preparation of" :,';: :nid€. Note that the wall of the blastula is com_tr:u.;i of distinct cells, the blastomeres. Identily theirmdei Spaces between blastomeres in the vegetal hemi-@re are shrinkage spaces produced during prepara_tr:,n ,r; r'our slides.:a,i-ine the animal hemisphere. It has the following;miir'neristics: (1) there is a heavily pigmented cortex,M.rr: trsrnentation being most intense at the animal polelmc ;:ading off progressively toward the vegetal pole;
Fig. 2.a, Prospective fate map of the frog blastula'
Th"e approximate site of blastopore formation is in-
30 Chaprer 2
be essential in many species to provide a space into
which certain groups of cells can move eithet en masse
or individual$ during gastrulation'
the blastoporal lips (see Chapter 6, Exercise 2'3)'
The locations of several areas (designated arbitrarily as
areas l--10 and25-27) before and during their involu-
tion over the dorsal and ventral blastoporal lips are
shown in Fig. 2.5. Allof these areas are located on the
surface at the blastula stage. Areas 1-5 have undergone
involution over the dorsqlblastoporal lip by the dorsal
lip gasffula stage. Similarly, areas 1-8 have.undergone
inuitrrriott ou"i th" dorsal blastoporal lip by the yolk
plug gastrula stage, and areas2l and26 have undergone
itru6tlntiott over the ventral blastoporal lip' The re-
maining numbered areas will undergo involution dur-
ing subsequent development' O1!:t cellular areas
,rrid"rgo involution over the lateral blastoporal lips in
a similar manner.
Fig.2.4shows the locations of several prospective areas'
soire of which undergo involution during gastrulation'
Some of the cells of the prospective endoderm and all
the cells of the prospective head mesenchlme and
prospective notochord involute over the dorsal blasto-
porui tip. Similarly, some of the cells of the prospec-
iive endoderm, some of the cells of the prospective
lateral plate mesoderm, and all the cells of the prospec'
tive sefrnental plate mesoderm involute over the later-
a/ lips"of the blastopore. The remaining cells of the
fro.p""ti"" lateral plate mesoderm' as well as some of
ifr" ""Ut
of the prospective endoderm, involute over the
ventralblastop-oral1ip. Only a relatively small number
of 'prospective endodermal cells undergo involution
over ttre Ulastoporal lips. Most prospective endodermal
cells remain relativeiy stationary during gastrulation
stages forming the yolk plug and floor of the archen'
ter6n (Fig.2.Sj. As areas involute, the prospective neur'
at plate aiO prospective ectoderm (epidermis) undergo
spieading, oi "piboly,
toward the blastopore to replace
aieas that have moved into the interior of the gastrula'
Obtain a slide containing sagittal sections of the dorsal
lip gastrula and select a section closely resembling Photo
Z'21 ldentrfy the blastopore' dorsal blastoporal lip' and
blastocoel. The blastopore represents the future cau-
dal end. The future cranial end lies directly opposite'
The blastopore opens into a narrow cavity, the primi-
tive gut, oiarchenteron. The floor of the archenteron
is foimed by yolk-filled endodermal cells' The cranial
end of the archenteron roof is formed from endoderm
because the first cells to involute over the dorsal and
lateral blastoporal lips, and thus to contribute to the wall
of the archenteron, are the cells of the prospective en'
doderm (Fig.Z.q- The remainder of the archenteron
roof at thi gastrula stage is formed by mesoderm'which
involutes over the dorsal and lateral blastoporal lips fol-
lowing involution of prospective endoderm' The meso-
derm"of the archenteron roof vill' later be covered by
endoderm, which migrates upward over the inner sur-
face of the mesoderm (see below)' Thus the archen-
teron is ultimately lined entirely by endoderm' Note
that the ectoderm in the frog consists of two layers: an
dicated by a circular dashed line'
l. Prospective ectoderm (epidermis)2. ProsPective neural Plate3. ProsPective notochord4. Prospective segmental plate mesoderm
5. Prospective lateral plate mesoderm
6. ProsPective head mesenchYme7. ProsPective endoderm
F. GASTRULATION
During gastrulation some of the cells originally locat-
eA on itt-e surface of the blastula turn inward or under-
go involution to move into the interior' These cells will
live rise to two germ layers: the endoderm, or innermost
iaver. and the mesoderm, or middle layer; cells that re-
-uirr ott the surface form the outermost germ layer'
the ectoderm. A depression, the blastopore, begins to
form just below the gray crescent as cells initiate invo-
tution @'ig. Z.3h). Simultaneously, a liplike structure, the
dorsal Lhstoporal lip, forms just above the blastopore'
With formati,on of the blastopore and dorsal blastopo-
ral lip, the blastula is transformed into a gastmla' Cells
contiiue to involute over the dorsal blastoporal lip with
further development, and involution progressively oc-
curs laterally and ultimately ventrally as well' This re-
sults in formation of a circular blastopore containing a
mass of yolk-filled endodermal cells called the yolk plug
@ig.2.3i). The circular blastopore is surrounded by con-
iioiorrt dorsal, tateral, and ventral blastoporal lips' The
directions of gastrulation movements can be altered ex-
perimentally,iesulting in exogastrulation, a process dur-
ine which surface cells move but fail to involute over
,:-t--Er"-f "
blastocoel
blastocoel
-site of dorsal
Frog Embryos 31
archenteron
dorsalblastoporaltr lip
dorsalblastoporal* rif,
blasto-pore
aichenteron
blastocoel
blastopore
yolk plug
blastopore
ventral\?
: i :siu1a Stagea
blastoporal lipIormatron
blastoporall lp
dorsal lip gastrula stageb
yolk plug gastrula stagev
*ilt''
,Il llu
l-5' Drawings of the cut surfaces of right halves of blastula and gastrula stages of tn" rrog[ *i.rr.otl ltrnal side; V ventral side; VP, vegetal pole; numbers 1-10 indicate specific areas of the blastula that un-
;11i1gr'p ' -'- 'rfution over the dorsal blastoporal lip; numbers 25-27 indicate specific areas that undergo involutiffi ;fi;;:",:":il#;:llil:"1l]o'Iu'[lon
nmm ryerficial) ectodermal layer and an inner (deep)uumdennal laver." lirli :T i slde containing sagittal sections of a yolk plugiltlllttfir-*: and select a section closely resembling photo: : ...r=niri_tr the yolk plug, a protruding mass of yolk_:L^rur .:-..jodermal cells located between the very promi_urlnrurr ilursal blastoporal lip above and the less prominentqruilmd blastoporal lip below. Also identify the archen-rfunlra - -ra-p1gd above the mass of poorly defined endo_ruLi*tr,& ;ells forming its floor. The yolk plug fills the*rmnrr:;,- to the archenteron, the blastopore. BeneathHiu r--;:Lenteron and separated from the latter by en_jrhrurir::,al cells is the irregularly shaped blastocoel. Thisilir,iilirirri -i later squeezed out of existence. Note that then'ululr::ed cortex now covers all cells except those oflie ' .-ft plug. The more rapidly dividing cells of thenmry*ttire ectoderm (epidermis) and prospective neur-u dllire undergo epiboly between the blastula stage andItrrL,s -i jranced gastrula stage, growing down over the',r dk-:l-ied cells of the vegetal hemisphere. Meanwhilq'iliru ,r-Lrri endodermal cells of the vegetal hemisphere'r',: iJnken inward to some extent and become re_lur:lrr-l;d to form the floor and ventrolateral walls oitrrrrr .L:;henteron. Most of the archenteron roof still con_mn; : i mesoderm at the yolk plug gastrula stage.ilirr- i. :,.'rmparison of gastrulation in vertebrates and in-eirL.r.atel see Chapter 1, Section H.
6- NEURUTATION-hrr :lodermal cells directly overlyirig the archenteron"ir'1r 19 r- rhe yolk plug gastrula stage constitute the neur-n unmoderm. The neural ectoderm is induced to thick-:n ,u :he neural plate. Induction of the neural plater,,o'f *= s'ith prospective mesodermal cells within the
dorsal lip of the blastopore, which send their induction- signal through the horizontal plane of the ectoderm.The initial signal is later reinforced by mesodermal cellswithin the roof of the archenteron, which send their in_duction signal vertically to the overlying ectoderm. Thegastrula is transformed into the neurula with formationof the neural plate.
Obtain a slide containing transverse sections of a neu_rula at the neural plate stage. Select a section closelyresembling Photos2.4,2.5 and identi{y the neural ptate.The roof of the archenteron at the neurula stage isformed by endoderm. Between the gastrula and neu_rula stages, endoderm migrated upward over the innersurface of the mesoderm that previously formed thearchenteron roof. Note that the dorsal mesoderm isnow organized into a midline rod of cells, the notochord,flanked by two bands of cellq the segmental plate meso-derm. This latter mesoderm is the source of the somites.The segmental plate mesoderm gradually merges lat_erally with the lateral plate mesoderm. The coelomforms within the lateral plate mesoderm.Obtain a slide containing transverse sections of a neu-rula at the neural fold stage and select a section close_ly resembling Photos 2.6,2.7.Identify the paired neuralfolds, which have formed at the lateral marsins of theneural plate, and the neural groove. The notochord, seg-mental plate mesoderm, and lateral plate mesoderm aremore clearly defined at this stage, and the endodermof the roof of the archenteron exists as a more distinctlayer. During subsequent development the neural foldswill fuse in the dorsal midline to close the neural srooveand thus establish the neural tube. Neurai creJt cellswill later formfrom the roof of the neural tube and giverise to a multitude of structures, including pigment cells(see Chapter 6, Exercise 2.4).
32 Chapter 2
H.d.ucedby the lens to form the transparent corneal ep'
ffi;t""i. At the level of the optic cups' sections cut
;;;;;;" ;tosencephalon (veniral$) continuous with
1. lntroduction the rnesencephalon (dorsatly)'
Fig.Z.rashows the general shape of th.e _b-ody at this The brain constricts into two separate parts as sections
stage, and Fig. Z.O ,t o*, ttt" ,t*itor", visible in a mid- are tracJ po1t"t-t1V Gtrofl 2J0)' The ventral part is
sagittal section. Familiarize yourself withthe spatial re the infundi:butum, the source of the posterior pituitary
lationships of these structuies before "*umioi"g
;"'iut gr""a t"""t"nvpophysis)' The dorsal part is the
transverse sections.
'uctures Detore ti^drruu - ir'o*n"*"prtoron ittt" tut*" *"tencephalon and mye'
turr""pt'uloi oittre'Urain;' Identify a solid ectodermal
2. Serial transverse sections rodaiaboutthislevel'justventraltotheinfundibulum'
Positionyoursrideonthemicroscopestageso'l'llTl *Ll;yi';*ffiiliJnT:T:xt#;'l'rlTf;tiew"A tlrougtr the microscope, each section
" ottll
t"Oi..* .t the anterior pituitary gland is continuous
"J u" rn PhotJ 2.8. Do not place microscope n:F'
'l'4t, ilffi;dermal invagination, the stomodeum' from
fi ir"i*a on slides to hold them in plac" ^ '!:::::': ;ffi; originated as an outgrowh' (See Photos 2'L0'
'iir*rnt r"rtions. E'xamine slides in anteroposterror se-
i .i-i"r ,rt" iocation of the stomodeum' The rudiment
A;;;;. unless directed otherwise (see Chapter z' ;irh" ;;fu pituitary gland and the stomodeum are
dection B). "orrtirrroo.
uia'section 6vel between those illustrated
a. Ecrodermar derivatives ;i::,X,,*?a?f)#ff-T3l,"",ffi:#iitrffffi:The first few sections cut through the tip of the head'
Gi"ni i1U, the adiesive glands (ventral suckers)' to
rdentify the rurg" pror"lnlph;d (ry,.T"i"t""t"4*l: ;i;;;rtd" of the stomodeum and extending through
and ailnceptralon of the brain), which is- enclosed by many sections.
'*,':H1"3iffi::ft$i:::ff)'ff'ilffi[i:ffi ]""i:'hi:13;111:'il1iT":t':*HitiJfifil*"r*3;:;1*H[::ff*"iil1"1"'ffi'li,T:['Jii?.i:i*f#L,T;1*=*:*';::*(i;#JilJi:"11fl :"ment. Tiy to identify a region-ot tfri"t"t"J
'ftit u""tt,,'"tuii*of ectodermalcells(neuralcrestcells) just
ectoderm on each side ventral to ttr" proseric*til above. each of them' These are the semilunar ganglia of
rhese thickenings are the nasar (orract'"ffi:ffii::li i*,*i:ru;t,t*Tffi|:'ffi' S^y#",::":);"1!{{{The nasul placodes eventually form the tt"iltt::]"f
-"itlu*, on each side, an epibranchial placode' will
:ilL:ffi:ii'"ilX'i?J#:l5if":#,ffy"'fi:ilffi1 fi::Filetothesegangria) continuetotracesec-
granules at the periph".ry of these placod; tJ""#;- - ti.on: ;;;;;;;t""0lolntitv the paired auditory vesi-
teristic of invaginating ectodermal iliffi; *:'$;u""ttorut"*fto.therhombencephalon(Photopresumably, the peripheial ends ot inuugiiliffi:,Jil z'ni'-rte"evesicles originated from thickenings of the
become narrowed, thus concentrating ,nJ piE-"", '1i';""l"O"tt"f layer'-which subsequent$ invaginat-
content, whereas the inner ends of the cens "riturg". "0.
rtr" u"oitory vlsicles later differentiate into the
Young newo n nt", oiinate from these placodes and inner ears'
producetheaxonsoitfieoractory(I)cranialnerves'Continuetotlacesectionsposterior$andnotethattheas well as the dendrites that function as orfactory re- ,r"rrrui,,rb" gradual$ 1T1:*.,
indicating the level of
ceptors (receptors ;;; d;;;e of smell). Nasaipla- tft" .pi""i"oiJ Gno.o 2'L4)' Examine the spinal cord
codes may not have yet developed.in some embryos. *rtnrrig5magniiication. Noteitscharacteristicthinroof
Identify the pinear gland (epiphysis), which originated "rd
fl;;;i;and its thick lateral walls' At about this
as a dorsal "uugi'i^tio"
from the p'o*"t'""phu1o"' l"""il;;;ttfy pigmented or nonpigmented cells lying
continue to trace sections posteriorly and identify the ao"uito trt" tpi"i "ota
and beneath the skin ectoderm;
optic cups (photo iil."r,^[n "ptic
cup is derived from these are neural crest cells (Photo 2'14)' As you ap-
a laterar evagination from the prosencephalon, which p.ou"t it "
rirral sections in your set' identify the dorsal
secondarily iruug-ui"Jut it, ufirra end forming a dou- ^fi"
#;;" rpi""r cord (Photos z'r5-2'18\' The loose
ble-layeredopticcup.Thethickenedlayeroftf,eopticcellsformingitscoreareneuralcrestcells,whichinducecupisthesensory'"ti"";thethinlayeristhepigment'formationofthisstructure'Notethatthebodynarrowsed retina. The optic cups are connected to the prosen- prog;lu"iv J
"u"oal,levels; identify the ventral fin
cephalonbytheopt icstalks. .Notethattheinnerut t r ' " " l . ,aaiendofthebody(Photo2.1.8). Ident i fytheectodermal layer adjacent to the sensory retina has pro"roa""-, a ventral invagination of skin ectoderm
thickened to form the lens placode @hoto 2'10)' The it'"t'it "1"*
sections anteriir to the ventral fin (Photo
"oui", pr,g^""ted layer of the ectoderm will laterbe in- 2'I7)'
Frog Embryos 33
I41aIJ
Fry. 2.6. Schematic drawing of a midsagittal section of a 4-mm frog embryo.
h. Endodermal derivativesR.eturn to the level of the nasal placodes (photo 2.8)nd trace sections posteriorly. Identify a small cavity--,rns beneath the prosencephalon (Photo 2.10). This'. the cranial end of the foregut; its walls are formed:"rm endoderm. The stomodeum has invasinated to-a ard the foregut, and the stomodeal ectodeim and the:",regut endoderm are in contact as the oral membrane.llris membrane later ruptttres to form the mouth open-mg. The outer part of the mouth is therefore lined bystomodeal ectoderm, and the inner part by foregut en-:oderm.
llhe foregut expands as the pharynx as sections are:raced posteriorly (Photo 2.12). (In a few embryos the-ateral walls of the pharynx may contact the skin ecto-:erm, which invaginates slightly to meet them. Such lo-;alized pharyngeal expansions are the pharyngealpouches.) Continue tracing sections posteriorly. Theioregut narrows and then forms a prominent ventraler-agination, the liver rudiment (Photo 2.13). The levelof the foregut that is continuous ventrally with the liverrudiment is the duodenum. The liver rudiment sepa-rates from the duodenum and then fades out a few sec-
\otochordSubnotochordal rod.\rtusHindgutYolk-filled endodermal cellsLateral plate mesodermLiver rudimentPericardial cavity
9. Heart (ventricle)10. Oral membraneI l . Stomodeum12. Rudiment of the anterior
pituitary gland13. Infundibulum14. Prosencephalon15. Pineal gland
16. Head mesenchyme17. Mesencephalon18. Rhombencephalon19. Pharynx20. Midgut21. Spinal cord22. Neural crest cells23. Dorsal fin
tions more posteriorly. The remaining portion of thegut is the midgut (Photos 2.14,2.15).It contains a smallcavity bounded dorsally by a thin layer of endodermalcells and ventrally by a large mass of yolk-filled endo-dermal cells. Continue to trace sections posteriorly fol-lowing the midgut. It gradually moves ventrally andenlarges somewhat as the hindgut (Photo 2.16). (Insome embryosthe endoderm of the hindgut fuses withthe ectoderm of the proctodeum in more posterior sec-tions to form the cloacal membrane. The cloacal mem-brane ultimately ruptures to form the anus.)
c. Mesodermal derivativesReturn to the level where the infundibulum first ap-pears (Photo 2.11) and trace sections posteriorly.Identify a group of mesodermal cells, the notochord,lying beneath the rhombencephalon (photo 2.I2). Thevacuolated condition ofthe notochord is apparently re-sponsible for its rigidity, allowing this structure to serveas a longitudinal supporting rod in young embryos.Quickly trace sections posteriorly, noting the changesthat the notochord undergoes. It is smaller at caudallevels than at more cranial levels, and vacuolization isprogressively reduced toward the caudal end (compare
$
II
34 Chapter 2
Photos 2.1'4-1'.18). The caudal end of the notochord is
iess deu"loped ai this stage because the notochord. de-
velops in craniocaudal sequence' In postenor sectlons
laentify the somites (Photos Z'15,2'16)'paired blocks
of mesoderm lying ventrolateral to the spinal cord' AIso
identrfy u ,rnutl Juster of mesodermal cells lying be-
neath ihe notochord in caudal regions' the subnoto'
cfrorAaf rod (hypochord) (Photos 2'I5-2'I7)' Its
developmental significance is unknown'
Return to the level at which the liver rudiment is con-
tinuous with the duodenum (Photo 2'13) and trace sec-
iion, port"riorly. The mesoderm ventrolateral to the
somitis is usuaily in the form of more or less distinct
"fitn"U vesicles (Photo 2'14)' The round vesicle on
each side farthest from the spinal cord is the pronephric
duct. Just above the latter, and often continuous with
ii, ut" ttt" pronephric tubules' Thb pronephric duct and
tubules oi eachside constitute the pronephric kidney'
which is functional in amphibian larvae' The pronephric
kidney when well formed causes the body to bulge lat-
erad as the PronePhric ridge'
The lateral plate mesoderm (often difficult to identify
as a distinciarea) lies ventral to each pronephric kid-
n"y, U"*""" ttre itin ectoderm and the endoderm' This
mesoderm is in the process of splitting into an outer
iuyer of somatic -"*d"ttt',
adjacent to the skin ecto-
derm, and aninner layer of splanchnic mesoderm' ad-
jaceni to the endoderm' If somatic and splanchnic
mesoderm have iormed, identify a space (or a series of
small spaces) between them, the coelom'
Return to the level at which the foregut first appears
(Photo 2.10) and trace sections posteriorly' Identify the
"ot otrorr".tt (bulbus cordis) region of the developing
heart lying beneath the pharynx (PhotoZ'lL)' The heart '
, enlarges -as
the ventricle in more posterior sections
@hotlo 2.13). Note that the heart consists of an inner
luj"r, the endocardium, surrounded by an outer'thick-
eilayer, the myocardium' Both these layers are derived
from splanchnic mesoderm ' (Splanchnic mesoderm will
Iater iontrrbute to a third layer of the heart: its outer-
most covering.) Identify a thin layer of cells enclosing
the heart, the parietat pericardium, deriv edftom somatic
mesoderm. The parietal pericardium is usually sepa-
rated from the skin ectoderm by a shrinkage space' The
space between the heart and the parietal pericardium
is the pericardiat cavity. The heart is.suspended within
the pe'ricardial cavity by a dorsal bridge of splanchnic
*"*d"t-, the dorsal mesocardium'
The major blood vessels are in early stages of forma-
tion at ihi, ti*", and they are thus difficult to identify
with certainty in most embryos' Two major blood ves-
sels can sometimes be identified' The first aortic arch'
es mainly lie ventrolateral to the pharynx (PhotoZ'I2)'
(They will soon estabtsh connections with the conotrun-
cus via a pair of blood vessels that will lie ventral to
the pharynx, the ventral aortae' These latter vessels can
.o*Ltit*. be identified at this stage') The dorsal end
oi "u"tt
first aortic arch is continuous with a blood ves-
sel lying dorsolateral to the pharynx' the dorsal aorta
fii6r.V.nl. The dorsal aortae extend caudad andfade
i"i ut uU""t the level of the midgut (Photo 2'14)'
3. Summary of the contributions of the"
;;il +rs to.structures present in the4-mm trog emnrYo
Ectodermadhesive glands
auditorY vesicles
corneal ePithelium
dorsal fin
infundibulum
lens PlacodesmesencePhalon
nasal Placodesneural crest cells
oPtic cuPs
oPtic stalks
pineal gland
proctodeum
prosencePhalon
rhombencePhalon
rudiments of the anterior pituitary gland
semilunar ganglia
stomodeum
sPinal cord
ventral fin
Mesodermconotruncus
dorsal aortae
dorsal mesocardium
first aortic arches
notochord
parietal pericardium
pronephric kidneYs
somites
subnotochordal rod
ventricle
ffiderm[ttuJr(:€-;[1
'linriyg-:
!]rnlt.li"{i_:
i.Iiii€,: :!,;lnlent
illtlttriiul-i:d
ffiotemm and endodermj t . . iL:. =_'mbrane
lmtir. --rhrane
Frog Embryos 35
2.20 and2.24).
The lips of the blastopore form during gastrula stages(Photos 2.25-2.27). First the dorsal tip torms, followedby the lateral lips, and finally the veniral lip. The yolkplug continues to occupy the blastopore thro-ughout gas_trulation.
The neural plate forms and rolls up into the neural fubeb_etween the gastrula stage and early embryo stage(Photos 2.28-2.30). Concomitant with iormation of theneural tube, the embryo begins to elongate craniocau_dally. This elongation is an obviou, f"itrrr" of devel_oping frog embryos (photos 2.3I-2.32). The neural tubeextends throughout the length of the embryo. It bulgeslaterad at its cranial end to form the developing eyesin conjunction with the overlying skin ectoderm. Notethat the skin ectoderm in the ernbryos illustrated inPhotos 2.31,2.32 is covered with ciliary tufts. Thesestructffes establish currents around the embryos as theybeat, circulating fluids. These structures als-o functionin primitive locomotory movements before swimmingbegins. In 4-mm embryos the skin ectoderm at the cra_nial end of the embryo has invaginated in the midlineforming the stomodeum. Caudally, the skin ectodermis attenuated, demarcating the dorsal and ventral fins(Photo 2.32).
It sC{!{NING ELECTRONffiffimoscoPYffinryryuu* ::;rming electron micrographs (photos 2.19_.i,,i,,,Il :r; T{-_: "- ou r-isualize the shapes of developing frog,,rr|llUflllru. wf :::?n'os three-dimensionally. Note at cleav_lrullw lfiilrrs'::i Fhotos 2.19-2.24) the positions and orien_iiilllllltullll0lllnri :d' deerage furrows separating blastomeres and;l|ilw "illm:::-ces in sizes of the micromeres andriiisu'rrFnrslf'es. Boundaries between blastomeres becomeirunlri,iNr ,u*erll
-:led as cleavage advances (compare photos
I TER,MS TO KNOWrilllrliiin.& iilfii;{I|': klorv the meaning of the following terms, which appeared inI)rilii]lilfll iinff lrt-, :5-
boldface in the preceding discussion of
epiboly
epibranchial placode
epidermis
epiphysis
exogastrulation
eyes
female pronucleus
fertilization
first aortic arches
first meiotic division
first polar body
floor plate
fluorescein-labeled dextranfollicles
follicle cells
foregut
gastrula
gastrulation
germ layers
unur,mi, r:ipophtSiS
u,Ltriluex;-: giands
i{m:[n"i* ::;misphere
u,lrmrrr;r nlle
lrmrfiE:: -r pituitary gland
{lllJt L.iu}
idflll:tli;::iron
riuruilr: - if-,- r esicles
&rlr, L>
lm.rUUSf -C,:el
'lrtiliffi : ni are s
llni.rtits'. -L'tlfe
lrui,u5|*.
lfiiiLtr-:Iion
:mil -r-;i-[s1
inil:r:r- c]; gOnadotropin
"- ; i , "* -" i -El l ]& i .JIL\
***,0 ua- furrows
cloacal membrane
cloacal valves
coelom
conotruncus
corneal epithelium
cortex of egg
deep ectodermal layer
dendrites
diencephalon
dorsal aorta
dorsal blastoporal lip
dorsal fin
dorsal lymph sac
dorsal mesocardium
duodenum
ectoderm
egg capsule
endocardium
endoderm
m36 Chapter 2
germinal vesicle
gray crescent
hindgut
holoblastic cleavage
horseradish peroxidase
hypochord
infundibulum
inner ears
inner ectodermal laYer
involution
lateral blastoPoral liPs
lateral plate mesoderm
lens placode
liver rudiment
macIomeres
male pronucleus
mature ovum
mesencePhalon
mesoderm
metaphase
metencePhalon
micromeres
midgut
morphogenetic movements
mouth opening
myelencephalon
myocardium
nasal cavities
nasal placodes
neural crest cells
neural ectoderm
neural folds
neural groove
neural plate
neural tube
neurohypophysis
neurula
notochord
nuclei
nuptial pads
olfactory (I) cranial nerves
olfactory placodes
olfactory receptors
oogenesis
oogonia
optic cups
optic stalks
oral membrane
ostium of oviduct
outer ectodermal laYer
ovaries
oviduct
ovulation
parietal pericardium
pericardial cavity
pharyngeal pouches
pharynx
pigment cells
pigmented retina
pineal gland
plasmalemma
posterior pituitary gland
primary oocyte
proctodeum
pronephric duct
pronephric kidney
pronephric ridge
pronephric tubules
prophase
prosencephalon
prospective ectoderm
prospective endoderm
prospective fate
prospective fate maP
prospective head mesenchYme
prospective lateral Platemesoderm
prospective neural Plateprospective notochord
prospective segmental Platemesoderm
rhodamine-labeled dextran
rhombencephalon
roof plate
rudiment of the anteriorpituitary gland
second meiotic division
second polar body
secondary oocytes
segmental plate mesoderm
semilunar ganglia
sensory retina
skin
skin ectoderm
somatic mesoderm
somites
sperm
spinal cord
splanchnic mesoderm
stomodeum
subnotochordal rod
superficial ectodermal laYer
surface ectoderm
telencephalon
theca folliculi externa
theca folliculi interna
total cleavage
trigeminal (V) cranial nerves
vegetal hemisphere
vegetal pole
ventral aortae
ventral blastoPoral liP
ventral fins
ventral suckers
ventricle of heart
vital dyes
vitelline membrane
yolk
yolk plug
young neurons
ilr38 Chapter 2
1. Pigmented cortex
2. Vitelline membrane
3. Blastocoel
4. Areaofgraycrescent
5. Shrinkage spaces
6. Nuclei
7. Blastomeres
8. Outer ectodermal laYer
9. Inner ectodermal laYer
@rrtt* 2./-2.5
Frog Embryos
felend10. Archenteron roof (mesoderm,
except at cranial end whereroof formed from endoderm)
11. Direction of epibolY
12. Dorsal blastoporal liP
13. Blastopore
14. Archenteron
15. Yolk-filled endodermal cells
16. Yolk plug
17. Ventral blastoporal liP
18. Neural plate
19. Skin ectoderm
20. Lateral plate mesoderm
21. Directions of cellular migrationto form endodermal roof ofarchenteron
22. Segmental plate mesoderm
23. Notochord
24. Archetteron roof (endoderm)
2.2t.
Photo 2.2. Frogdorsal lip gastrula (sagittal section). D, Dorsal side;Y Ventral side.
Photo 2.1. Frog blastula (sagittal section).AP, Animal pole;D, Dorsal side;VVentral side;VP;Vegetal pole'
Frog Embryos 39
Mtmto 2.3. Frog yolk plug gastrula (sagittal section). D, Dorsal side; V ventral side.
Flimtos 2-4t 2.5. Frog neural plate neurula (transverse section). Photo 2.5 is anenlargement of the dorsal:i-nion of Photo 2.4.
It|40 Chapter 2
1. Neural groove
2. Skin ectoderm
3. Lateral plate mesoderm
4. Archenteron
5. Yolk-filled endodermal cells
6. Neural fold
7. Outer ectodermal layer
8. Inner ectodermal layer
@/ntu*2.C2J)
Frog Embryos
9. Segmental plate mesoderm
10. Notochord
11. Archenteron roof (endoderm)
12. Pineal gland
13. Prosencephalon
14. Nasal placode
15. Mesencephalon
16. Pigmented retina of the oPticcup
17. Sensory retina of the oPtic cuP
18. Optic stalk
19. Rudiment of the anterior Pi-tuitary gland
20. Head mesenchyme
21. Stomodeum
Photos 2.6, 2.7. Frog neural fold neurula (transverse section). Photo 2.7 is an enlargement of the dorsal
portion of Photo 2.6.
Frog Embryos 41
Fhotos2.B,2.9.4-mmfrogembryoserial t ransversesect ions,, , ,
ilr-42 Chapter2
L.
2.
3.
4.
5.
6.
7.
8.
Mesencephalon
Future corneal ePithelium
Lens placode
Rudiment of the anterior Pi-tuitary gland
Foregut
Oral membrane
Adhesive gland
Stomodeum
@hrtug2.1O-2./3
Frog Embryos
'Yge/"{9. RhombencePhalon
10. Skin ectoderm
11. Semilunar ganglion
12. Infundibulum
13. Auditory vesicle
14. Notochord
15. Dorsal aorta
1-6. Pharynx
17. First aortic arch
1-8. Conotruncus
19. Duodenum
20. Liver rudiment
21. Dorsal mesocardium
22. Peicardial cavitY
23. Endocardium of ventricle
24. Myocardium of ventricle
25. P arietal Pericardium
26. Shrinkage space
Photos 2.1O, 2.1"1.posterior sequence.
ffifrogembryoserialtransversesectionsnumberedinanteriorto
Frog Embryos 43
lflflhmnnri I l l. 2.13.)i l l l l l | l i lufl ' - r:*--OC€.
Continuation of 4-mm frog embryo serial transverse sections numbered in anterior to
44 Chapter 2
1. Roof plate
2. Neural crest cells
3. Spinal cord
4. Floor plate
5. Dorsal aorta
6. Pronephric tubule
7. Pronephric duct
@hrbs,2./1-2./B
Frog Embryos
fgle',il
8. Pronephricridge
9. Midgut
10. Yolk-filled endodermal cells
11. Skin ectoderm
12. Liver rudiment
13. Dorsal fin
1-4. Somite
15. Notochord
16. Subnotochordalrod
17. Hindgut
l-8. Proctodeum
19. Ventral fin
2.15
Photos 2.14, 2.15. Continuation of 4-mm frog embryo serial transverse sections numbered in anterior to
posterior sequence.
Frog Embryos 45
Wflrllmlilrt : n m-1.18. Continuation of 4-mm frog embryo serial transverse sections numbered in anterior tol l l l l l l l l l l :1"1" * , : - - ; lC€.
46 Chapter2
@Dtu*2./3-2.21
Frog Embryos
1. Cleavage furrow
2. Blastomere
3. Micromere (blastomere)
4. Macromere (blastomere)
ofafrogtwo-ce1lstage(viewedfromtheside).AP,Animalpo1e;
VP, Vegetal Pole.
ffigelectronmicrographofafrogfour-ce1lstage(viewedfromanimalpoleandside).
Frog Embryos 47
lrlllllllltttnrrlt! -ln S;anrring electron micrograph of a frog eight-cell stage (viewed from animar pole and side).
rlllllllrnmrn . " a.
lllllllillililiiit"' I Il : -: :
S;rnning electron micrograph of a frog sixteen-cell stage (viewed from animal pole and side; alli:= :tot visible).
S;anning electron micrograph of a frog early blastula (viewed from animal pole and side).tllllllllnnrnrum * I l
,UfillN*n,i* :-:.1" S;anning electron micrograph of a frog late blastula (viewed from animal pole).
48 Chapter2
1. Yolk plug
2. Dorsal blastoporal liP
3. Lateral blastoporal liP
4. Archenteron
5. Ectoderm
6. Roof of archenteron
7. Blastocoel
-&otot 2.2o-2.3O
Frog Embryos
8. Blastopore
9. Future brain level of neuralplate
10. Future spinal cord level ofneural plate
11. Neural fold
12. Skin ectoderm
13. Future brain level of neuralgroove
14. Future spinal cord level ofneural groove
15. Future brain level of earlYneural tube
16. Future spinal cord level ofearly neural tube
Photo 2.25. Scanning electron micrograph of a frog dorsal lip gastrula. D, Dorsal side.
i
tiiI
l7III
t]
IIlIIl "L
photo 2.26. Scanning electron micrograph of a parasagittal slice through a frog yolk plug gastrula. D, Dorsal
side:V.Ventral side.
50 Chapter 2
1. Neural tube
2. Ciliary tufts
@tttu*2.3/-2.32
Frog Embryos
fgretu{
3. Developing eye
4. Dorsal fin
5. Ventral fin
6. Stomodeum
I
I
II,.i
It
h'6II
tPhoto 2.31. Scanning electron micrograph of a frog early embryo (dorsolateral view). R, Right side.
Photo 2.32. Scanning electron micrograph of a frog 4-mm embryo (lateral view). H, Head (cranial) end;!Tail (caudal) end.
rII
Ii Amphibian Development: Frog 4-mm Embryo (Stage 1g) g.l
olfactory pit prosencephalon
epiphysismesencephalon
optic cup
lens placode
pharynx
branchial cleft2
otic vesicle
branchial cleft 3
pericardial cavity
rhombencephalon
pronephric area
somites
spinal cord
notochord
endoderm
- :'!:r-.. r!stonrocleurn
adgland
l ivercl ivc'r ' t iL'u Iu nT
Figure 103
4-mm frog embryo (stage 18), whole mount (60K.
: .
bud
82 Amphibian Development: Frog 4-'nm Embryo (Stage 18)
stomodeum
adenohypoPhYsis
oral evagination
adhesive gland
thyroid rucliment
pericardial cavitY
heart
endoderm
vcntt'al mesoderm
Figure 104
epiphysis
prosencephalon
mesencephalon
infundibulum
rhombencePhalon
notochord
pharynx
liver diverticulum
spinal cord
midgut
subttotochordal rod
hindgut
tail bud
proctocleum t/
1
| +-mm frog embryo (stage '18), sagittal section (60K'
,lilflI
t
I--
Amphibian Development: Frog lO-rnm Tadpole (Stage 24) 101
l ip
nror-rth
pharynx
thyloid
heart
l iver
intestine
cloaca
anus
Figure 130
1O-mm frog tadpole (stage 24), whole mount (55X.
exterL.tnl nares
internal rrares
letina
lens .
otic vesicle
pronephros
stontacl-r
colcl
notochorcl
l tai l f i rr
. l 02 Amphibian Development; Frog 1O-mm Tadpole (Stage 24)
lip
tooth
pharynxthyroid
infundibulum
heart
intestine
myotomes
telencephalon
diencephalon
metencephalou
myelencephalon
trachea
esophagus
notochord
dorsal aorta
myotomes
hindgut
spinal cord
tail fin
Figure 13L
' l O-mm frog tadpole (stage 24), sagittal section (35K.
104 Amphibian Development: Frog .t O-mm Tadpole (Stage 24)
telencephalon
olfactoly organ
tootir
Figure 133
l0 nrm Irog tacl l to le (stage 24), I ranSvcrse sect ion Lhrough the ol factory organ (5OX).
diencephalon
pignrentccl layer of l 'ct i tra
sensory layel of retina
cranial cartilage
cranial cartilatge
lyrnph sinus
Figure 134
10-nrm frog tadpole (stage 24), transverse section through the eyes (SoX).
rnuscle thyroid
Amphibian Development: Frog 10-nrnr Tadpole (Stage 24\ 1Os
myelencephalon
velar plate
atrium
notochorcl
opcrcLrla
Figure 135
lO-rrrnr l rog tar lpolc (sf . . tge 24), t t .anSVerSc sect ion through the heart (5OX).
ganglion, cranial nerve VII
gangl ion, cranial r rcrve V
pharynx
myelencephalon
ganglion, cranial nerve IX
pharynx
glottis
sinus venosus
opercular chamber
ot ic vesic le
notochorcl
gi l ls
intestine
Figure 136
lO-nrm frog taclpole (stage 24), transverse section througlr the glottis (sox).
106 AmPhibian DeveloPment:Frog ' l O-mm TaclPole (Stage 24)
myelen lion, cranial nerve X
pfonephfos
lung
esophagus
opercular chamber
liver
intestine
gallbladder
Fig=ure 137 .
1O-mnr frog tadpole (stage 24)' transverse section through ihe pronephros (50X'
esophagusmYeie
notochordpronePhros
Figure 138
;;;;.;;oo"'"tlttn" 24)' transverse section through the liver (sox'
Amphibian Development: Frog 1O-mm Tadpole (Stage 24) 107
spinalganglion
notoch
Figure 139
mesonephros
mesonephric duct
hindgut
peritoneal cavity
i O-mrn frog tadpole (stage 24), transverse section through the mesonephros (50rc.
spinal cord
spinal ganglion
notochordi
dorsal aorta
somite
hindleg bud
loaca
\ Figure 140\
' l O-mm frog tadpole (stage 24), transverse section through the cloaca (50K.
fin
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CLEAVACE STAG"ES
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. 'NEUITAT Sl AGES
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NIUITAL PLN'I 'E STACL:
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68 Arnphibian Deveiol : r r rentrFtog Cleovfrge/GnslrLrloliotl (St(r[|s$ *. I I l
outcr l i lyet .ol 'ectclc lernr
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Figure 87
Frog erlbryo, late gastr"ula (stage
lQn
1einner lnyer archerrtcron r.ocl [ 'of 'cctocle rnr (chclcla rncsodcr- lu)
rulcherr lcr 'on (glst rocoel)
b l l ts to l rorc
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10), sagi t la l sect ior . r (100X)
2Frog EmbryosA. Introduction 25
B. How to Use Serial Sections 25
C. Oogenesis and Fert i l ization 27
D. Formation of the Cray Crescent
E. Cleavage and Blastulation 29
F. Castrulation 30
C. Neurulation 3' l
H. 4-mm Frog Embryos 32
l. Scanning Electron Microscopy 35
J. Terms to Know 35
K. Photos 2.1-2.32 37
2B