blastoderm 



SECTION op 



BLASTULA 



Fig. 4. — Cleavage of the three types of chordate eggs and formation of the blastula. A, 

 Amphioxus; 1-4, cleavage; 5, external view of the blastula; 6, section of the blastula. Note that 

 the cells of the vegetal hemisphere are but slightly larger than those of the animal hemisphere, the wall 

 of the blastula is one cell-layer in thickness, and the blastocoel is large. B, amphibian; 1-4, cleavage; 

 5, blastula; 6, section of the blastula. The cells of the vegetal hemisphere are considerably larger than 

 those of the animal hemisphere, the wall of the blastula is at least two cell layers in thickness, and the 

 blastocoel is smaller and displaced dorsally. C, reptile or bird egg with meroblastic cleavage; i-j, 

 cleavage; 4, median sagittal section of the blastula. Only the germinal disk cleaves, forming a disk 

 of cells — the blastoderm — resting on the yolk; a slight slit between this and the yolk represents the 

 blastocoel. {A and B, 1-5, from Parker and Haswell's Textbook of Zoology, after Hatschek, courtesy 

 of the Macmillan Company; B6 from Prentiss and Arey's Textbook of Embryology, courtesy of the 

 W. B. Saunders Company.) 



