GENERAL EMBRYOLOGY. 



155 



Distribution of the Types of Cleavage. Of the four types of 

 cleavage mentioned the superficial one has an interest from the 

 point of view of the systematist, since it occurs exclusively in the 

 arthropods. The other modes of cleavage are distributed as fol- 

 lows: the discoidal has heen observed in the majority of the 

 vertebrates and in the most highly organized molluscs, the 

 A 



FIG. 103.- Discoidal cleavage of the egg of a cephalopod (Loligo pealii). 

 (After Watase.) 



TIG. 104. Superficial cleavage of an insect egg (Pieris cratcegi). (After Bobretzky.) 

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

 to form the blastoderm; C\ formation of the blastoderm. 



cuttlefishes, while the equal and the unequal cleavage can be found 

 in all the groups of the Metazoa. 



Blastula. Sometimes during the first stages of segmentation, 

 sometimes later, there is usually formed a cavity, the cleavage or 

 sea mentation cavity, between the cells in the interior of the egg; 

 with the progress of development this cavity becomes continually 

 larger (fig. 100, B). Around it the cells lie in the form of a one- 

 layered or of a many-layered epithelium and form the blastoderm; 

 hence the name for this stage, Nastodermic vesicle, or, briefly, 

 Uastula. The more yolk there is present, the smaller is the 

 cleavage cavity; in centrolecithal eggs with superficial cleavage it 

 is entirely absent. 



