GENERAL EMBRYOLOGY 



145 



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

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

 cavity, {arcliicivle) bet^Yeen the cells, in the interior of the egg; with the 

 progress of development this cavity becomes larger (figs. loi, IV; 103). 

 Around it the cells lie in the form of a one- or many-layered epithelium 



C 



Fig. ios — Discoida! cleavage of the egg of a cephalopod, Loligo pealli (after 



Watase). 



Fig 106. — Superficial cleavage of an insect egg, Pier is cralirgi (after Bobretzky). 

 A, division of the cleavage nucleus; 5, 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, blastodermic 

 vesicle, or blastula. 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 Metazoa. 



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 



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