44 WALTER HEAPB. 



which they form an almost continuous layer, and they are 

 distinctly darker stained than are the deeper placed, more 

 columnar cells. They are generally wide at the top, ending 

 below in a wedge-shaped base, which grows downwards between 

 two of the columnar cells lying beneath. Some of the cells 

 are, however, more flattened, possessing no downward pro- 

 longation, and some are more columnar, having little or no 

 expanded upper surface ; indeed, there are cells in all stages 

 of transition, between the flattened outer layer cells of the 

 previous stage and the columnar cells of the future epiblast 

 plate (vide fig. 49, t. c). 



Kolliker's valuable paper contains most careful descriptions 

 and drawings, which, however, appear to me to be capable of 

 a very different interpretation from that put forward by him ; 

 in fact, they appear to me to be strongly confirmatory of my 

 own views. He states that the large nucleated plates which 

 are visible in surface views of young areas split up in older 

 embryos into small polygonal areas without nuclei. Now, I 

 would venture to suggest that the disappearance of the nuclei 

 of these large outer layer plates can be fully accounted for by 

 their migration downwards among the cells of the inner mass 

 (vide fig. 49): and the apparent breaking up of the large cells 

 may be explained by the actual appearance on the surface of 

 the epiblast plate, of the polygonal ends of the columnar cells 

 of which it is now composed. 



Stage C. 



The Formation of the Mesoblast. 



The middle germinal layer has two distinct sources : in the 

 first place it arises from the epiblast and hypoblast at the hind 

 end of the embryonic area, in the structure known as the 

 primitive streak ; and, secondly, from the hypoblast alone in 

 the anterior region of the area in front of the primitive streak. 



