36 



HOW ANIMALS DEVELOP 



egg-cell. This process is known as the cleavage of the 

 tgg. The details vary according to the amount of 

 yolk which the tgg contains. Eggs with very little yolk 

 cleave into equal parts ; those with rather more yolk 



NO 

 YOLK 



SOME 

 YOLK 



MUCH 

 YOLK 



EGG 



CLEAVAGE STAGE 



BLASTULA 



Fig. 6. — Diagrammatic sections of eggs, cleavage stages and blastulae 

 with various amounts of yolk 



cleave into small cells at the top or animal pole 

 and larger cells at the bottom where the yolk is 

 collected. Eggs with a great deal of yolk, such as 

 birds' eggs, do not cleave throughout their whole 

 mass : only the patch of non-yolky cytoplasm cleaves, 

 forming a plate of little cells swimming on the 

 surface of the main mass of uncleaved yolk. Such 



