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 ecto- 

 dermic cells (fig. 56 ah} 

 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 cuhical 

 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 a 

 single layer of much 

 flattened scale-like cells, 

 which are closely united 

 into a thin membrane. 

 Toward the periphery 

 they become somewhat 

 larger and more poly- 

 gonal (fig. 57), and here, 

 at some distance inside 

 the free margin of the 

 outer germ-layer, they 

 become merged in the 

 white yolk {dw), which 

 is abundantly provided 

 with yolk-nuclei (dh) in, 

 the region of the transi- 

 tion. This region of the. 



