82 



COLLEGE ZOOLOGY 



one side of the egg (as in birds), the process 

 is spoken of as meroblastic or discoidal 

 cleavage (Fig. 41). If cleavage is restricted 



to a layer of cytoplasm around the entire 

 egg as in insects, it is spoken of as superfi- 

 cial cleavage (Fig. 41). 



Blastula 

 (surface) 



Blastula 

 (section) 



Egg with slight yolk (starfish) 



Gastrula 

 (section) 



o 



.2 o 



to U 



Egg with moderate yolk (frog) 



Egg with much yolk (reptiles, birds, most fishes,- some invertebrates as the squid) 



Yolk 



Yolk 



Centra! yolk mass (most arthropods) 



Figure 41. Top three rows: types of cleavage; blastulae, and gastrulae of vertebrates. A, archen- 

 teron (gastrocoel). B, blastocoel (segmentation cavity). Bottom row: cleavage; blastulae and 

 gastrulae of arthropods. 



Blastula 



As cleavage advances, a cavity becomes 

 noticeable in the center of the egg (Fig. 41), 

 enlarging as development proceeds until the 

 whole resembles a hollow rubber ball, the 

 rubber being represented by a single layer of 

 cells. At this stage the embryo is called a 

 blastula, the cavity the blastocoel (segmen- 

 tation cavity), and the cellular layer the 



blastoderm. The blastula resembles some- 

 w^hat a single colony of Volvox (Fig. 22). 

 The blastula wall in some cases is more than 

 one cell thick, and the segmentation cavity 

 may be lacking. 



Gastrula 



The cells on one side of the blastula begin 

 to invaginate (infold) (Fig. 41) into the 



