124 F. M. BALFOUR AND F. DEIGHTON. 



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 

 beyond. 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 show up in surface views. 

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

 primitive streak is shown in a longitudinal section of a ducVs 

 blastoderm of this stage (fig. d). The same figure also shows 

 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, 

 shows that the hypoblast is here thickened, and that its upper 

 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 meso- 

 blast 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 

 it is very much deeper. One of the latter is represented in 

 fig. G. It has here the appearance of a narrow slit, and 

 sections of it give the impression of the mesoblast originating 

 from the lips of a fold ; in fact, the whole structure appears like 

 a linear blastopore, from the sides of which the mesoblast is 

 growing out; and this as we conceive actually to be the true inter- 

 pretation of the structure. Other cases occur in which the 

 primitive groove is wholly deficient, or at the utmost represented 

 by a shallow depression along the median axial line of a short 

 posterior part of the primitive streak. 



We may now pass to the consideration of the part of the area 

 pellucida in front of the primitive streak. 



We called attention to a change in the character of the hypo- 

 blast cells of this region as taking place at the end of the last 

 stage. During the very early part of this stage the change in 

 the character of these cells becomes very pronounced. 



What we consider to be our earliest stage in this change we 



