THIRD PERIOD OF DEVELOPMENT. 71 



from all other hypoblast cells by a somewhat more 

 granulated appearance, and a larger nucleus. We 

 shall further see that these cells, which always mark 

 the posterior pole, constitute in the formation of the 

 mesoblast its posterior extremity. AVe will there- 

 fore describe them as the polar mesoblast cells. 



In the transverse sections, also made from the 

 embryos of this stage, it can be seen that the body 

 wall is everywhere composed only of two cell layers 

 (Fig. 71). The characteristic flattening of the dorsal 

 surface, which introduces the important changes of 

 the next stages of the development, is well seen in 

 the transverse section (Fig. 71). 



From this stage on begin changes which run their 

 course with great rapidity. These are scarcely to 

 be thoroughly investigated in the living object. 

 The examination must further be made with suitable 

 re-agents of special preparation. Farther, a metho- 

 dical examination of transverse sections is absolutely 

 necessary. 



THE LIVING OBJECT. 



I will in my description proceed from those observa- 

 tions which can be made on the living object. 



The first change which we see is a deep depres- 



