RENEWED STUDY OF GERMINAL LAYERS OV THE CHICK. 183 



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

 The thinning out of the median portion of the mesobhist of the 

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

 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. 



"U hat we consider to be our earliest stage in this change wc 

 have only so far met with in the duck, and we have figured a longi- 

 tudinal and median section to show it (PI. XIII, fig. d). The 

 hypoblast (/li/) has become a thick layer of somewhat cubical cells 

 several rows deep. These cells, especially in front, are characterised 

 by their numerous yolk spherules, and give the impression that 

 part of the area pellucida has been, so to speak, reclaimed from the 

 tarea opaca, Fosteriorly , at the front em of the primitive stnah^ 



