SECT. 6] 



OF THE EMBRYO 



919 



Amphioxus 



Petromyzon 



Triton ... 



Rana 



Amia 



Adpenser 



Ceratodus 



Salamandra 



Lepidosiren 



The nature of the yolk is also believed to influence to a large extent 

 the form of cleavage. Thus yolk with large formed elements probably 

 necessitates unequal divisions, and Hertwig was able by centrifuging, 

 to make the frog's egg, which normally divides totally but unequally, 

 divide partially. 



Table no. 



Absorption 



The special absorption methods necessitated by partial cleavage 

 are shown in Table no. In the simplest type, shown by Salamandra, 

 the macromeres gradually hand over their contents to the rest of 

 the embryo. The appearance of merocytes and yolk-cells, situated 



