Summary

• Here is the bottom line:– Every animal starts out as one cell (zygote).– This cell divides (Cleavage) into two cells, then

those 2 cells divide and become 4 cells, then 8, and so forth

– Cleavage starts at the animal pole and proceeds towards the vegetal pole.

– This continues until you get a hollow ball of cells• It is at this stage that the embryo is called a blastula

Summary

• Next, invagination of the cells on one side of the blastula develops a hollow center within the embryo that becomes the gut, or archenteron.

• It is at this stage that the embryo is called a gastrula.

• The hole that formed on the side as a result of the invagination is called a blastopore.

Starfish (Asterias) early embryology

Blastopore Archenteron (early gut)

Ectoderm

Endoderm

Nucleolus

Nucleus

Single cell(haploid gamete)

Nucleusnot visible

Single diploid cell

(Early cleavage =10 or fewer cells)

OR

-ovum, an unfertilized egg; nucleus and nucleolus are visible-zygote, a fertilized egg (diploid); the nucleus is not visible

-2 cells, the product of mitosis of the zygote, cell size is halved, then again to 4 cells, etc., to blastula

-gastrula forms as a result of the infolding of cells to form a gut

Archenteron

FROG EMBRYOLOGY

Ovary

Early cleavage

Gastrula

EggsEarly cleavage

Blastula

Neurula

Micromeres

Macromeres

Animal pole

Vegetal pole

Vegetal pole

Micromeres

Macromeres

Animal poleBlastocoel

Blastocoel(the smaller cavity)

Archenteron

EctodermMesoderm

EndodermBlastopore

Yolk plug

Neural groove(forming neural

tube)

Gut

Somites

Notochord

(Early cleavage =16 or fewer cells)

Notes

• Neural tube becomes the dorsal nerve cord

• Notochord becomes the backbone (vertebral column)

CHICK EMBRYO Eye

Brain

Ear (hard to see)

Heart

Neuraltube

Somites

Misc follows

Starfish

• Egg Zygote

Archenteron