Life History And Life History And EmbryogenesisEmbryogenesis

From: ChingFrom: Ching

Preparedby: Geonyzl L AlviolaPreparedby: Geonyzl L Alviola

Development and Development and AncestryAncestry

Embryology is also needed in zoologyEmbryology is also needed in zoologyAfter the publicationAfter the publication of Darwin’s of Darwin’s origin origin

of the species- phylogeny and of the species- phylogeny and ontogeny become the subject of ontogeny become the subject of observation and speculation.observation and speculation.

Ontogeny repeats phylogeny while,Ontogeny repeats phylogeny while,

embryology reveals ancestry.(Haeckel)embryology reveals ancestry.(Haeckel)

De Beer –that recapitulation is not De Beer –that recapitulation is not the only relationship between the only relationship between embryos and ancestors. embryos and ancestors. (meaning……)(meaning……)

Gametes and FertilizationGametes and Fertilization

Gametes are the mature sex cellsGametes are the mature sex cells

sperm = malesperm = male

egg = femaleegg = femaleEach carries a haploid number of Each carries a haploid number of

chromosomeschromosomesSperm and egg would vary from Sperm and egg would vary from

classes of vertebratesclasses of vertebrates

Classification of EggClassification of Egg

It was classified based on the It was classified based on the amount of yolk:amount of yolk:

Types of egg based on amount of Types of egg based on amount of yolk:yolk:

1. microlecithal 4. isolecithal1. microlecithal 4. isolecithal

2. mesolecithal2. mesolecithal 5. telolecithal 5. telolecithal

3. macrolecithal 3. macrolecithal

designed more for providing designed more for providing nutrient sources to the nutrient sources to the developing young than for developing young than for movement movement

contains yolk that consists of contains yolk that consists of lipids and protein for lipids and protein for nutrients, along with enzymes nutrients, along with enzymes needed to initiate needed to initiate development development

Microlecithal EggsMicrolecithal Eggs

Small eggs with little yolk.Small eggs with little yolk.and the young hatch quickly as a and the young hatch quickly as a

result result characteristic of the protochordates characteristic of the protochordates

and eutherian mammals and eutherian mammals

Mesolecithal EggsMesolecithal Eggs

(characteristic of lampreys (characteristic of lampreys and amphibians) and amphibians)

have an intermediate amount have an intermediate amount of yolk, and the young hatch of yolk, and the young hatch at a later stage of at a later stage of development development

Macrolecithal EggsMacrolecithal Eggs

(characteristic of fishes, (characteristic of fishes, reptiles, birds and reptiles, birds and monotremes)monotremes)

have a large amount of yolk, have a large amount of yolk, and the young hatch at an and the young hatch at an even later stage even later stage

Isolecithal EggsIsolecithal Eggsdistribution of yolk can be distribution of yolk can be

even through the eggeven through the egg

Telolecithal EggsTelolecithal Eggsyolk concentrated in one part of yolk concentrated in one part of

the egg - the area with less yolk the egg - the area with less yolk and prominent haploid nucleus and prominent haploid nucleus as the as the animal poleanimal pole and the area and the area with more yolk as the with more yolk as the vegetal vegetal polepole

Group type of egg type of cleavage blastulablastula cavity

Amphioxus, mammals

isolecithal, oligolecithal

or microlecithal (little yolk)

holoblastic (or complete): initial cleavage planes

extend through egg)

sphere with single layer

wall

large, central sphere

amphibians, lampreys, lungfish,

paddlefish, sturgeon

mesolecithal (moderate

yolk)holoblastic

sphere with layered

wall

small sphere

reptiles telolecithal

meroblastic (incomplete): initial cleavages planes

limited to animal pole, the region opposite the

yolk)

cell disk on surface of

yolk, bilaminar

flat space between epiblast

and hypoblast

The egg was also surrounded by The egg was also surrounded by a delicate a delicate vitelline membranevitelline membrane

SpermSperm are are: : • • extremely small cells that extremely small cells that lack most of their cytoplasm lack most of their cytoplasm • designed to travel through an • designed to travel through an aquatic medium (either aquatic medium (either internal or external) to reach internal or external) to reach the egg cell the egg cell • travel by movements of one • travel by movements of one or more flagella that propel it or more flagella that propel it toward the egg toward the egg

• • all sperm consist of all sperm consist of three basic three basic piecespieces: :     - the     - the headhead, which contains the , which contains the genetic material and is capped genetic material and is capped by the by the acrosomeacrosome (cap) at the (cap) at the apex that contains enzymes apex that contains enzymes needed for the sperm to needed for the sperm to penetrate the egg penetrate the egg     - the     - the middlemiddle piece contains piece contains the primary power source of the primary power source of mitochondria that fuel the mitochondria that fuel the movements of the movements of the tailtail piece. piece.

FertilizationFertilizationAt fertilization, enzymes in the acrosome At fertilization, enzymes in the acrosome

of the sperm help to penetrate the eggof the sperm help to penetrate the egg requires that the sperm break through requires that the sperm break through

the plasma and vitelline membrane the plasma and vitelline membrane surrounding the egg surrounding the egg

to prevent more than one sperm from to prevent more than one sperm from penetrating the egg (penetrating the egg (polyspermypolyspermy), the ), the egg undergoes a cortical reaction to egg undergoes a cortical reaction to bring the sperm head into the interior of bring the sperm head into the interior of the egg and change the vitelline the egg and change the vitelline envelope to form the fertilization envelope to form the fertilization membrane membrane

CleavageCleavage

Just after fertilization the Just after fertilization the zygotezygote (fertilized egg) undergoes (fertilized egg) undergoes cleavagecleavage (mitotic cell divisions) and becomes (mitotic cell divisions) and becomes subdivided into smaller cells - the subdivided into smaller cells - the gross arrangement of cells differs gross arrangement of cells differs greatly among vertebrates, greatly among vertebrates, depending on the amount of yolk in depending on the amount of yolk in the egg: the egg:

Holoblastic cleavageHoloblastic cleavage

occurs when the cleavage furrows pass occurs when the cleavage furrows pass through the entire egg through the entire egg • cleavage can either be equal, where • cleavage can either be equal, where the resulting cells contain the same the resulting cells contain the same amount of yolk, or unequal, in which amount of yolk, or unequal, in which some cells contain more yolk than some cells contain more yolk than others: others:     - equal cleavage occurs in     - equal cleavage occurs in microlecithal eggs microlecithal eggs     - unequal cleavage occurs in     - unequal cleavage occurs in mesolecithal eggs mesolecithal eggs

• • cleavage results in the formation

of a ball of cells (blastomeres) surrounding an internal cavity (blastocoel)

Meroblastic cleavageMeroblastic cleavage

occurs more in macrolecithal eggs occurs more in macrolecithal eggs • cleavage takes place only in a disk • cleavage takes place only in a disk at the animal pole at the animal pole • the cleavage furrows do not extend • the cleavage furrows do not extend into the yolk into the yolk • results in the formation of the • results in the formation of the blastodisk that lies on the top of the blastodisk that lies on the top of the yolk yolk

Cleavage of Microlecithal Cleavage of Microlecithal EggsEggs

This picture is of the unfertilized egg. It can be differentiated from the zygote by the presence of a large, conspicuous nucleus (large arrow) with obvious nucleolus (smaller arrow) and by the lack of a fertilization membrane. Next…..

This shows the zygote (fertilized cell). It is recognized by the presence of the fertilization membrane (arrows) surrounding it and the peripheral, fluid-filled perivitelline space.

Next….

Cleavage of Mesolecithal Cleavage of Mesolecithal EggsEggs

Cleavage of Megalecithal Cleavage of Megalecithal EggsEggs

From this enlarged cross-sectional view of the chick blastula, some important anatomical details are visible and labelled:

Ao = Area opaca Ap = Area pellucida 1 = Blastocoel 2 = Subgerminal spa

All blastulas have two main components--the blastoderm and blastocoel. The blastoderm is the sheet of cells on the outside of the blastula. The blastocoel is the cavity on the inside.

is characterized by cell movement is characterized by cell movement and reorganization within the embryo and reorganization within the embryo ((morphogenetic movementsmorphogenetic movements) to ) to the interior of the embryo, forming the interior of the embryo, forming three primary germ layers:three primary germ layers:

ectoderm, ectoderm, mesoderm, mesoderm, and endodermand endoderm. .

GastrulationGastrulation

The cells migrate inward at the The cells migrate inward at the blastoporeblastopore, , which forms, or is close to, the location of which forms, or is close to, the location of the anus in the adult the anus in the adult • the ectoderm forms the outer tube of the • the ectoderm forms the outer tube of the embryo embryo • the endoderm is an inner tube that forms • the endoderm is an inner tube that forms the alimentary canal and all its derivative the alimentary canal and all its derivative organs organs • the mesoderm lies between these two • the mesoderm lies between these two layers. layers.

This blastopore-to-anus developmental This blastopore-to-anus developmental pathway is found in Chordata, pathway is found in Chordata, Hemichordata, Echinodermata (starfish, Hemichordata, Echinodermata (starfish, sea urchins, sea cucumbers, etc.), sea urchins, sea cucumbers, etc.), uniting these groups into a uniting these groups into a monophyletic group called the monophyletic group called the DeuterostomesDeuterostomes. The plesiomorphic . The plesiomorphic condition, found in the condition, found in the ProtostomesProtostomes, is , is for the blastopore to become the for the blastopore to become the mouth. mouth.

NeurulationNeurulation

occurs at or near the end of occurs at or near the end of gastrulation and transforms the gastrulation and transforms the gastrula into a neurula by gastrula into a neurula by establishing the central nervous establishing the central nervous system system • the ectoderm gives rise to neural • the ectoderm gives rise to neural folds flanking a neural groove along folds flanking a neural groove along an axis from the blastopore toward an axis from the blastopore toward the future headthe future head

OrganogenesisOrganogenesis

Differentiation and derivation - Differentiation and derivation - Organogenesis:Organogenesis: After the production of the nerve After the production of the nerve tube, differentiation of the germ tube, differentiation of the germ layers occurs rapidly, and layers occurs rapidly, and organogenesis begins, in which the organogenesis begins, in which the primary tissues differentiate into primary tissues differentiate into specific organs and tissuesspecific organs and tissues

EndodermEndoderm

- Endoderm gives rise to the epithelium of - Endoderm gives rise to the epithelium of the alimentary tract, to structures derived the alimentary tract, to structures derived from the pharyngeal pouches such as from the pharyngeal pouches such as parathyroid glands, thymus gland, parathyroid glands, thymus gland, Eustachian tube and middle ear cavity (not Eustachian tube and middle ear cavity (not the ossicles), and to structures that develop the ossicles), and to structures that develop as an evagination of the gut, such as the as an evagination of the gut, such as the thyroid gland, lungs or swim bladder, liver, thyroid gland, lungs or swim bladder, liver, gall bladder, pancreas, and urinary bladder. gall bladder, pancreas, and urinary bladder.

MesodermMesoderm - becomes organized into - becomes organized into three regions: three regions:

1. the epimere (dorsal mesoderm), 1. the epimere (dorsal mesoderm),

2. mesomere (intermediate 2. mesomere (intermediate mesoderm)mesoderm)

3. hypomere (lateral mesoderm)3. hypomere (lateral mesoderm). .

      

EpimereEpimerethe somites constitute most of the dorsal the somites constitute most of the dorsal mesoderm and have three regions: mesoderm and have three regions:

• • dermadermatome - forms the dermis of the tome - forms the dermis of the mid-dorsal skin mid-dorsal skin

• • sclerotomesclerotome gives rise to the gives rise to the vertebraevertebrae

• • myotomemyotome forms skeletal muscles forms skeletal muscles other than those of the gill archesother than those of the gill arches

MesomereMesomere::

      -gives rise to the kidney tubules, -gives rise to the kidney tubules, excretory organs, and reproductive excretory organs, and reproductive ducts. ducts.

HypomereHypomere::

- lateral-plate mesoderm is confined to lateral-plate mesoderm is confined to the trunk and is divided into: the trunk and is divided into:

somatic mesoderm (parietal somatic mesoderm (parietal peritoneum) and peritoneum) and

splanchnic mesoderm splanchnic mesoderm

- visceral peritoneum, mesenteries, - visceral peritoneum, mesenteries, heart and associated structures, heart and associated structures, lymphatic system, gonads and visceral lymphatic system, gonads and visceral musclesmuscles

EctodermEctoderm - gives rise to: - gives rise to:

Neural tube Neural tube Epidermis and associated glands Epidermis and associated glands Neural crest and its derivatives: Neural crest and its derivatives:

migrate through the embryo, giving migrate through the embryo, giving rise to a diversity of structures rise to a diversity of structures

summarysummary

Thank youThank you

One derived characteristic found in vertebrates is One derived characteristic found in vertebrates is the the formation of neural crest cellsformation of neural crest cells • ectodermally derived • ectodermally derived • develop along the top of the neural tube as the • develop along the top of the neural tube as the neural folds close neural folds close • most neural crest cells change into • most neural crest cells change into mesenchyme, an embryonic tissue that consists mesenchyme, an embryonic tissue that consists of star-shaped cells from all three germ layers of star-shaped cells from all three germ layers • develop into the visceral skeleton (i.e. gill • develop into the visceral skeleton (i.e. gill arches, some of which will develop into jaws), arches, some of which will develop into jaws), pigment cells, sensory and postganglionic pigment cells, sensory and postganglionic neurons, the dentine-producing cells of teeth, neurons, the dentine-producing cells of teeth, Schwann cells that help protect neurons, and Schwann cells that help protect neurons, and bony scales bony scales

Differentiation and derivation - Organogenesis:Differentiation and derivation - Organogenesis: After the production of the nerve tube, differentiation of the germ layers occurs After the production of the nerve tube, differentiation of the germ layers occurs rapidly, and organogenesis begins, in which the primary tissues differentiate into rapidly, and organogenesis begins, in which the primary tissues differentiate into specific organs and tissues (Fig. 5.17). specific organs and tissues (Fig. 5.17). * * EndodermEndoderm - Endoderm gives rise to the epithelium of the alimentary tract, to - Endoderm gives rise to the epithelium of the alimentary tract, to structures derived from the pharyngeal pouches such as parathyroid glands, thymus structures derived from the pharyngeal pouches such as parathyroid glands, thymus gland, Eustachian tube and middle ear cavity (not the ossicles), and to structures gland, Eustachian tube and middle ear cavity (not the ossicles), and to structures that develop as an evagination of the gut, such as the thyroid gland, lungs or swim that develop as an evagination of the gut, such as the thyroid gland, lungs or swim bladder, liver, gall bladder, pancreas, and urinary bladder. bladder, liver, gall bladder, pancreas, and urinary bladder. * * MesodermMesoderm - becomes organized into three regions: - becomes organized into three regions: the epimere (dorsal the epimere (dorsal mesoderm), mesomere (intermediate mesoderm), and hypomere (lateral mesoderm), mesomere (intermediate mesoderm), and hypomere (lateral mesoderm)mesoderm). .     -     - EpimereEpimere: The somites constitute most of the dorsal mesoderm and have three : The somites constitute most of the dorsal mesoderm and have three regions: regions:

• • dermadermatome - forms the dermis of the mid-dorsal skin tome - forms the dermis of the mid-dorsal skin • • sclerotomesclerotome gives rise to the vertebrae gives rise to the vertebrae • • myotomemyotome forms skeletal muscles other than those of the gill arches     - forms skeletal muscles other than those of the gill arches     - MesomereMesomere: gives rise to the kidney tubules, excretory organs, and reproductive : gives rise to the kidney tubules, excretory organs, and reproductive ducts. ducts.     -     - HypomereHypomere: lateral-plate mesoderm is confined to the trunk and is divided into : lateral-plate mesoderm is confined to the trunk and is divided into somatic mesoderm (parietal peritoneum) and splanchnic mesoderm (visceral somatic mesoderm (parietal peritoneum) and splanchnic mesoderm (visceral peritoneum, mesenteries, heart and associated structures, lymphatic system, gonads peritoneum, mesenteries, heart and associated structures, lymphatic system, gonads and visceral muscles) and visceral muscles)

* * EctodermEctoderm - gives rise to: - gives rise to: • Neural tube • Neural tube • Epidermis and associated glands • Epidermis and associated glands • Neural crest and its derivatives: migrate through the embryo, • Neural crest and its derivatives: migrate through the embryo, giving rise to a diversity of structures giving rise to a diversity of structures • Ectodermal placodes: localized thickenings that sink below the • Ectodermal placodes: localized thickenings that sink below the surface and give rise to sensory neurons and sensory structures: surface and give rise to sensory neurons and sensory structures: olfactory placodes, forming the olfactory sacs; lens placodes, for olfactory placodes, forming the olfactory sacs; lens placodes, for the lens of the eye; otic placodes, to become the membranous the lens of the eye; otic placodes, to become the membranous labyrinth; a group of placodes that contributes neurons to the labyrinth; a group of placodes that contributes neurons to the sensory ganglia of cranial nerves V, VII, VIII, IX, and X; and last, sensory ganglia of cranial nerves V, VII, VIII, IX, and X; and last, placodes that form the neuromasts of the cephalic and lateral line placodes that form the neuromasts of the cephalic and lateral line canals canals Sources of energy during development and Sources of energy during development and extraembryonic membranesextraembryonic membranes The The cleidoic eggcleidoic egg is an important derived trait among many is an important derived trait among many vertebrates and enabled tetrapods to be independent of water. In vertebrates and enabled tetrapods to be independent of water. In just considering the macro- and mesolecithal species, we know just considering the macro- and mesolecithal species, we know that both contain a moderate to large amount of yolk for the that both contain a moderate to large amount of yolk for the embryo to use as an energy source. In the cleidoic egg, water and embryo to use as an energy source. In the cleidoic egg, water and oxygen are obtained through diffusion. oxygen are obtained through diffusion.

Extraembryonic membranes vary in complexity among the Extraembryonic membranes vary in complexity among the vertebrates:vertebrates: Yolk sacYolk sac - forms around the yolk, and connects to the embryo via the yolk - forms around the yolk, and connects to the embryo via the yolk stalk to provide nutritional support during development stalk to provide nutritional support during development - amniotes and anamniotes differ in their yolk sacs - amniotes and anamniotes differ in their yolk sacs     fish have a trilaminar yolk sac, with an extraembryonic coelom     fish have a trilaminar yolk sac, with an extraembryonic coelom that surrounds the yolk that surrounds the yolk     birds and reptiles possess a bilaminar yolk sac (consisting only     birds and reptiles possess a bilaminar yolk sac (consisting only of endoderm and splanchnic mesoderm). of endoderm and splanchnic mesoderm). Shell membrane/shellShell membrane/shell - is formed only in the cleidoic egg of amniotes - is formed only in the cleidoic egg of amniotes - surrounds the embryo, yolk and albumins (egg white) and - surrounds the embryo, yolk and albumins (egg white) and protects it protects it - provides a surface for diffusion of oxygen - provides a surface for diffusion of oxygen     Allantois    Allantois - acts as a compartment for storage of nitrogenous - acts as a compartment for storage of nitrogenous excretory products such as uric acid, and may remain after birth excretory products such as uric acid, and may remain after birth or hatching as the urinary bladder. or hatching as the urinary bladder.     Amnion    Amnion - surrounds the embryo, and is filled with amniotic - surrounds the embryo, and is filled with amniotic fluid to cushion the embryo fluid to cushion the embryo     Chorion    Chorion - surrounds the amnion and yolk sac - surrounds the amnion and yolk sac

Mammalian developmental modificationsMammalian developmental modifications • the mammalian egg does contain some yolk, but it is microlecithal and • the mammalian egg does contain some yolk, but it is microlecithal and isolecithal - requires that the embryo implant quickly in order to obtain isolecithal - requires that the embryo implant quickly in order to obtain more nutrients from the mother. more nutrients from the mother. • early cleavage in mammalian embryos followed by the blastocyst stage, • early cleavage in mammalian embryos followed by the blastocyst stage, of which the outer layer of cells is called the trophoblast. The inner cell of which the outer layer of cells is called the trophoblast. The inner cell mass of the blastocyst will go on to form the embryo. mass of the blastocyst will go on to form the embryo. • during implantation in the uterus, the placenta is formed, which is a • during implantation in the uterus, the placenta is formed, which is a structure for physiological exchange between the fetus and the mother. structure for physiological exchange between the fetus and the mother. The placenta consists of both a maternal contribution (endometrium of the The placenta consists of both a maternal contribution (endometrium of the uterus) and fetal contribution (trophoblast), which is believed to be used as uterus) and fetal contribution (trophoblast), which is believed to be used as an immunological barrier that prevents rejection of the fetus (and its an immunological barrier that prevents rejection of the fetus (and its paternal chromosomes) by the mother. The shape of the placenta varies paternal chromosomes) by the mother. The shape of the placenta varies depending on the species. depending on the species. • the inner cell mass of the blastocyst develops into the blastodisk (similar • the inner cell mass of the blastocyst develops into the blastodisk (similar to that in chickens). Early stages of development of the mammalian to that in chickens). Early stages of development of the mammalian embryo, such as primitive streak stage, neurulation and germ layer embryo, such as primitive streak stage, neurulation and germ layer differentiation, are similar to that occurring in chickens and reptiles. differentiation, are similar to that occurring in chickens and reptiles. • the primary difference found in mammals is the development of the • the primary difference found in mammals is the development of the umbilical cord - contains allantois and yolk sac as well as circulatory umbilical cord - contains allantois and yolk sac as well as circulatory system structures that connect the embryo to the placenta. system structures that connect the embryo to the placenta.

Ontogeny and PhylogenyOntogeny and Phylogeny The notion of a parallel between the stages of development (The notion of a parallel between the stages of development (ontogenyontogeny) and ) and the evolutionary history of adults (the evolutionary history of adults (phylogenyphylogeny) predates the acceptance of ) predates the acceptance of evolution. It was thought that there was a Scala Naturae, a "Ladder of evolution. It was thought that there was a Scala Naturae, a "Ladder of Nature" or "Scale of Being" for living things, which could be arranged in a Nature" or "Scale of Being" for living things, which could be arranged in a sequence, as if on the rungs of a ladder. The highest rung was viewed as a sequence, as if on the rungs of a ladder. The highest rung was viewed as a stage of perfection. Likewise, it was generally noted that the ontogeny of an stage of perfection. Likewise, it was generally noted that the ontogeny of an individual consisted of a series of stepwise stages, and it was natural to individual consisted of a series of stepwise stages, and it was natural to assume a connection between the two. assume a connection between the two.

Carl Von BaerCarl Von Baer: made a number of general conclusions about development : made a number of general conclusions about development called Von Baer's laws: called Von Baer's laws:

1. In development from the egg the general characters appear before the 1. In development from the egg the general characters appear before the special characters. special characters. 2. From the more general characters the less general and finally the special 2. From the more general characters the less general and finally the special characters are developed. characters are developed. 3. During its development an animal departs more and more from the form of 3. During its development an animal departs more and more from the form of other animals. other animals. 4. The young stages in the development of an animal are not like the adult 4. The young stages in the development of an animal are not like the adult stages of other animals lower down on the scale, but are like the young stages of other animals lower down on the scale, but are like the young stages of those animals. In other words, a chick embryo would be stages of those animals. In other words, a chick embryo would be recognizable at an early stage as Vertebrata, but not any particular subtaxon. recognizable at an early stage as Vertebrata, but not any particular subtaxon. Later, it would be recognizable as Aves, and finally, it would be recognizable Later, it would be recognizable as Aves, and finally, it would be recognizable as a as a Gallus domesticusGallus domesticus. Therefore, the ontogenetic stages do not run par . Therefore, the ontogenetic stages do not run par

Ernst HaeckelErnst Haeckel: believed that the adult stages of the chain of : believed that the adult stages of the chain of ancestors are repeated during the ontogeny of the ancestors are repeated during the ontogeny of the descendants, but that these stages are crowded back into the descendants, but that these stages are crowded back into the earlier stages of ontogeny. Thus, ontogeny is an abbreviated earlier stages of ontogeny. Thus, ontogeny is an abbreviated version of phylogeny. Haeckel claimed that the gill slits of version of phylogeny. Haeckel claimed that the gill slits of human embryos were literally the same structures of human embryos were literally the same structures of ancestral adult fishes, that were pushed back into the early ancestral adult fishes, that were pushed back into the early ontogeny of humans by an acceleration of development in ontogeny of humans by an acceleration of development in lineages. In other words, the sequence of ontogenies was lineages. In other words, the sequence of ontogenies was condensed, and new features were added by terminal condensed, and new features were added by terminal addition. addition.

Von Baer, in contrast, argued that the gills slits are not the Von Baer, in contrast, argued that the gills slits are not the adult stages of ancestors; rather they are simply a stage adult stages of ancestors; rather they are simply a stage common to the early ontogeny of all vertebrates. That is, common to the early ontogeny of all vertebrates. That is, evolution proceeds from "undifferentiated homogeneity to evolution proceeds from "undifferentiated homogeneity to differentiated heterogeneity"; from the general to the differentiated heterogeneity"; from the general to the specific. Von Baer's theory requires only that organisms specific. Von Baer's theory requires only that organisms differentiate; Haeckel's requires a change in developmental differentiate; Haeckel's requires a change in developmental timing. timing.

Biogenetic LawBiogenetic Law Both of these ideas preceded Darwin's theory of evolution Both of these ideas preceded Darwin's theory of evolution and were re-read in the light of Darwin. Although Darwin and were re-read in the light of Darwin. Although Darwin favored Von Baer, Haeckel's ideas became more accepted. favored Von Baer, Haeckel's ideas became more accepted. Haeckel's theory came to be known as the theory of Haeckel's theory came to be known as the theory of recapitulation or the recapitulation or the biogenetic law - Ontogeny biogenetic law - Ontogeny recapitulates Phylogenyrecapitulates Phylogeny. This was an attractive idea, . This was an attractive idea, because it gave biologists a way of reading phylogeny because it gave biologists a way of reading phylogeny directly from ontogeny. directly from ontogeny.

The biogenetic law eventually lost popularity with the rise The biogenetic law eventually lost popularity with the rise of experimental embryology and Mendelian genetics. of experimental embryology and Mendelian genetics. Embryology showed that many varieties of change in Embryology showed that many varieties of change in developmental timing were possible, and that different developmental timing were possible, and that different parts of the organism might differ in rates of development; parts of the organism might differ in rates of development; Mendelian genetics showed that genes could effect Mendelian genetics showed that genes could effect changes at any stage of development, and that terminal changes at any stage of development, and that terminal addition was not the only possibility. addition was not the only possibility.

   

During neurulationDuring neurulation • the chordamesoderm that will go to form the notochord • the chordamesoderm that will go to form the notochord induces neural plate formation, which is the first stage in induces neural plate formation, which is the first stage in the formation of the neural tube. the formation of the neural tube. • characterized in most vertebrates by three stages • characterized in most vertebrates by three stages     - during the neural plate stage, the ectoderm on the     - during the neural plate stage, the ectoderm on the dorsal side of the embryo overlying the notochord thickens dorsal side of the embryo overlying the notochord thickens to form the neural plate to form the neural plate     - at the neural fold stage, the thickened ectoderm folds,     - at the neural fold stage, the thickened ectoderm folds, leaving an elevated area along the neural groove. The leaving an elevated area along the neural groove. The neural fold is wider in the anterior portion of the vertebrate neural fold is wider in the anterior portion of the vertebrate embryo, which is the region that will form the brain. embryo, which is the region that will form the brain.     - during the neural tube stage, the neural folds move     - during the neural tube stage, the neural folds move closer together and fuse - the neural groove becomes the closer together and fuse - the neural groove becomes the cavity within the neural tube, which will later be capable of cavity within the neural tube, which will later be capable of circulating cerebrospinal fluid that aids in the function of circulating cerebrospinal fluid that aids in the function of the central nervous system. the central nervous system.