The Development of the Chick 105 



of cells of various sizes and shapes. Between 

 these two layers is sometimes seen, at an 

 early period, before the egg is laid, a very 

 small cleft-like space, the segmentation cavity, 

 corresponding to the large and distinct cavity 

 of that name which was seen in the frog's 

 egg. At a somewhat later period may be 

 seen a more distinct cavity, the subgerminal 

 cavity, (eg) lying between the lower layer cells 

 and the yolk. 



In the centre of the blastoderm, the cells 

 of the lower layer are few and scattered, while 

 around the periphery they are more numer- 

 ous and form a comparatively thick layer 

 (Fig. 36). It is this difference in the thick- 

 ness of the different regions of the blasto- 

 derm that produces the distinction into an area 

 opaca and an area pellucida, when the blasto- 

 derm is viewed from above. 



Owing to the extreme delicacy of the blasto- 

 derm, it is difficult to obtain sections that will 

 show the above points. 



During incubation the blastoderm continu- 

 ally increases in size until it completely en- 

 closes the yolk. In this growth the area 

 opaca increases much more rapidly than the 

 area pellucida, and retains its circular outline, 



