RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 863 



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

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

 tions 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 

 interpretation of the structure. Other cases occur in which the 

 primitive groove is wholly deficient, or at the utmost repre- 

 sented 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 

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

 longitudinal and median section to shew it (PI. 43, fig. D). The 

 hypoblast (hy) has become a thick layer of somewhat cubical 

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

 characterized by their numerous yolk spherules, and give the 

 impression that part of the area pellucida has been, so to speak, 

 reclaimed from the area opaca. Posteriorly, at the front end of 

 the primitive streak, the thick layer of hypoblast, instead of being 

 continuous with the flattened hypoblast under the primitive streak, 

 falls, in the axial line, into the mesoblast of the primitive streak 

 (PI. 43, fig. D). 



In a slightly later stage, of which we have specimens both of 

 the duck and chick, but have only figured selected sections of a 

 chick series, still further changes have been effected in the con- 

 stitution of the hypoblast (PI. 44, Ser. H, I and 2). 



Near the front border of the area pellucida (i) it has the 

 general characters of the hypoblast of the duck's blastoderm just 

 described. Slightly further back the cells of the hypoblast have 

 become differentiated into stellate cells several rows deep, which 

 can hardly be resolved in t/ie axial line into hypoblast and meso- 

 blast, though one can fancy that in places, especially laterally, 

 they are partially differentiated into two layers. The axial 



