62 ARTHUR DRNDY. 



conspicuous beneath the embryo after the blastoderm has been 

 removed from the yolk (figs. 2, 16, 19, *S'. E. 31.), but it takes 

 no part in the development of the embryo itself. As far as I 

 can judge from the scanty literature at my disposal, it corre- 

 sponds to the secondary endoderm or yolk layer of Will (13) in 

 the gecko and other vertebrates. 



Above the segmentation cavity the lower layer outside the 

 embryo is a multiple layer of irregular cells with little or no 

 yolk in their meshes (fig. 4). At Stage C it appears to pass 

 gradually into the lower layer of the area opaca, but in some 

 sections of later stages the transition appears very sudden, 

 there being a distinct germinal wall at the edge of the area 

 opaca (fig. 14). 



In the embryo itself the epiblast is much thickened, and its 

 cells are arranged in a multiple layer. It is especially thick 

 in the region of the head-fold (fig. 4, H. F.), where the 

 epiblastic cells appear to be rapidly proliferating. Further 

 back it is thinner, and its cells distinctly prismatic, forming a 

 medullary plate (fig. 4, M. P.), which passes behind into the 

 very thick, undifferentiated cellular mass of the primitive 

 streak. Beneath the epiblast of the embryo there is a thick 

 layer of small rounded cells which also passes into the primi- 

 tive streak, from which, indeed, it cannot be histologically 

 distinguished, and from which it appears to have been derived 

 by forward growth and proliferation (fig. 4). In the region of 

 the head-fold this layer passes into the lower layer of the area 

 pellucida, but whether the cells of the latter actually extend 

 into the embryo at this stage (C) is very doubtful. 



Posteriorly the cells of the primitive streak pass into the 

 prismatic epiblast of the area pellucida above, while below 

 this they are continued backwards almost to the edge of the 

 area opaca as a thick compact layer of small rounded cells, in 

 no way distinguishable from the primitive streak itself. This 

 layer in turn becomes continuous with the original lower layer 

 of the blastoderm behind the embryo. 



It appears to me that we must regard the lower layer of the 

 embryo itself at this stage (C) as the mesoblast derived from 



