862 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 



The mesoblast is attached in the median line to the epiblast. 

 Laterally it extends outwards to the edge of the area pellu- 

 cida, and in older embryos may even form a thickening beyond 

 the edge (fig. G). Beneath the denser part of the mesoblast, and 

 attached to the epiblast, a portion composed of stellate cells 

 may in the majority of instances be recognized, especially in the 

 front part of the primitive streak. We believe these stellate 

 cells to be in the main directly derived from the more granular 

 cells of the previous stage. The hypoblast forms a sheet of 

 flattened cells, which can be distinctly traced for the whole 

 breadth of the area pellucida, though closely attached to the 

 mesoblast above. 



In sections we find that the primitive streak extends back 

 to the border of the area pellucida, and even for some distance 

 bayond. The attachment to the epiblast is wider behind; but 

 the thickness of the mesoblast is not usually greater in the 

 median line than it is laterally, and for this reason probably the 

 posterior part of the streak fails to shew up in surface views. 

 The thinning out of the median portion of the mesoblast of the 

 primitive streak is shewn in a longitudinal section of a duck's 

 blastoderm of this stage (fig. D). The same figure also shews 

 that the hypoblastic sheet becomes somewhat thicker behind, 

 and more independent of the parts above. 



A careful study of the peripheral part of the area pellucida, 

 in the region of the primitive streak, in older embryos of this 

 stage, shews that the hypoblast is here thickened, and that its 

 upjjer part, i.e. that adjoining the mesoblast, is often formed 

 of stellate cells, many of which give the impression of being 

 in the act of passing into the mesoblast above. At a later 

 stage the mesoblast of the vascular area undoubtedly receives 

 accessions of cells from the yolk below; so that we see no 

 grounds for mistrusting the appearances just spoken of, or for 

 doubting that they are to be interpreted in the sense suggested. 



We have already stated that during the greater part of the 

 present stage a groove, known as the primitive groove, is to be 

 found along the dorsal median line of the primitive streak. 



The extent to which this groove is developed appears to be 

 subject to very great variation. On the average it is, perhaps, 

 slightly deeper than it is represented in Ser. I, 5. In some cases 



