98 



EMBRYOLOGY. 



yolk where the latter 

 has not yet undergone 

 division into entodermic 

 cells. In the form of 

 its cells it is, in every 

 respect, in sharp con- 

 trast with the inner 

 layer. While the ccto- 

 dermic c?lls (fig. 56 ak) 

 attain their greatest 

 height in the middle 

 of the germ-disc, they 

 gradually decrease in 

 height toward the mar- 

 gin, and undergo a 

 transition into cubical 

 and finally into flat- 

 tened elements (fig. 57). 

 The reverse is the case 

 with the inner germ- 

 layer ; the latter has 

 now become converted in 

 the middle of the germ- 

 disc (fig. 56 ik] into n 

 single layer of much 

 flattened scale-like cells, 

 which are clos?ly united 

 into a thin membrane. 

 Toward the p-rij 

 they become somewhat 

 larger and more poly- 

 gonal (fig. 57), and h-iv. 

 at some distance inside 

 the free margin of the 

 outer germ-layer, they 

 become merged in iln- 

 whit- yolk ('/*/). which 

 is abundantly provi 1 > <1 

 with yolk-nurli i (<lk) in 

 t In region of the transi- 

 tion. Thi-> ivgion of the 



