GENERA L EMBU YOL G T. 



155 



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

 cleavage meutioned tlie superficial one has an interest from the 

 point of view of the S3'stematist, since it occurs exclusively in the 

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

 lows : the discoidal has been observed in the niajorit}' of the 

 vertebrates and in the most highly organized molluscs, the 

 A „ C 



y 



I'IG. lOo.— Disroidal clfcavas-Ce of tlif c-^cu' "f a ceplialopod (L'diiji) pcalii). 

 (After Watase.) 



Fig. 104.-&uperhcial cleavage of an inaeot egg (Picris crat ryi) ( After I tjietzk-\ ) 

 .1, division of the cleavage nucleus ; JB, movement of the nuclei t ) the t trii htrv 

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



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

 in all the groujis of the Metazoa. 



Blastula. — Sometimes during the first stages of segmentation, 

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

 segmentation caoUy, between the cells in the interior of the ego-; 

 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, Uastodennic vesicle, or, bncflv, 

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

 cleavage cavity; in centrolecithal eggs with superficial cleavage it 

 is entirely absent. 



