1 1 2 Vertebrate Embryology 



" In Z the splanchnopleure has completely invested the 

 yolk-sac, but at the lower pole of the yolk is still con- 

 tinuous with that peripheral remnant of the somatopleure 

 now called the serous membrane. In other words, the 

 age of the mesoblast has been carried all round the 

 Ays) except just at the lower pole. 



" In J/ the cleavage has been carried through the pole 

 itself; the peripheral portion of the splanchnopleure 

 forms a complete investment of the yolk, quite uncon- 

 nected with the peripheral portion of the somatopleure, 

 which now exists as a continuous membrane lining the 

 interior of the shell. The yolk-sac {ys) is therefore 

 quite loose in the pleuroperitoneal cavity, being con- 



