GENERAL EMBRYOLOGY 



145 



Blastula. Sometimes during the first stages of segmentation, some- 

 times later, there is usually formed a cavity, the cleavage or segmentation 

 cavity, (archiccele) between the cells, in the interior of the egg; with the 

 progress of development this cavity becomes larger (figs. 101, IV; 10^). 

 Around it the cells lie in the form of a one- or many-laye r ed epithelium 



A 



B 



' 



- 





FIG. 105. Discoidal cleavage of the egg of a cephalopod, Loligo pealii (after 



Watase). 



B 



FIG 106. Superficial cleavage of an insect egg, Pieris cratii^i al'a-r Bnlnvt/.ky). 

 .1, division of the cleavage nucleus; B, movement of the nuclei to the periphery to 

 form the blastoderm; C, formation of the blastoderm. 



and form the blastoderm; hence the name for this stage, blastodcrmic 

 vesicle, or blastida. The more yolk present, the smaller is the cleavage 

 cavity; in eggs with superficial cleavage it is entirely absent. 



4. Formation of the Germ-layers. 



Gastrula. Besides the blastula there is a second stage of development, 



the gastrula or the two-layered embryo, common to all the Meta/oa. 



This stage is understood easiest in eggs which have an equal cleavage 



(fig. 107, B) ; here it has the form of a double-walled cup with a wider or 



10 



