734 



LIFE : OUTLINES OF GENERAL BIOLOGY 



the fertilised ovum is usually a ball of cells. But when the yolk is 

 very abundant, as in reptiles and birds, a disc of cells— a discoidal 



Fig. 117. 



Various Types of Animal Ova (A-D), showing in (i) the undivided ovum, 

 in (2) the segmented ovum, becoming a blastula or morula stage, in 

 (3) a more advanced stage, in (4) the gastrulation. Thus A would represent 

 the sequence in a starfish, B in a crustacean, C in a tunicate, and D in 

 a frog. 



blastoderm — is formed at the upper pole of the great ball of nutritive 

 material, which it gradually surrounds. 



