RENEWED STUDY OF GERMINAL LAYERS OV THE CHICK, 125 



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

 tudinal and median section to show it (PI. VII^ fig. d). The 

 hypoblast (Jiy) 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. Vosteriorly , at the front end of the primitive streaky 

 the thick layer of hypoblast^ instead of being continuous toith the 

 flattened hypoblast under the primitive streak, falls, in the axial 

 line, into the mesoblast of the primitive streak (PI. VTI, 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. YIII, Ser. h, 1 and 2). 



Near the front border of the area pellucida (1) 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, %ohich 

 can hardly be resolved in the 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 sheet 

 of stellate cells is continuous laterally with cubical hypoblast 

 cells. 



As the primitive streak is approached an axial prolongation 

 forwards of the rounded and closely- packed mesoblastic 

 elements of the primitive streak is next met with, and at the 

 front end of the primitive streak, where this prolongation unites 

 with the epiblast, it also becomes continuous with the stellate 

 cells just spoken of. In fact, close to the end of the primitive 

 streak it becomes difticult to say which mesoblast cells are directly 

 derived from the primitive layer of hypoblast in front of the primi- 

 tive streak, and which from the forward growth of the mesoblast 

 of the primitive streak. There is, in fact, as in the earlier stage, 

 a fusion of the layers at this point. 



Sections of a slightly older chick blastoderm are represented 

 in PI. VIII, Ser. i, 1, 2, 3, 4 and 5. 



Nearly the whole of the hypoblast in front of the primitive 

 streak has now undergone a differentiation into stellate cells. 

 In the second section the products of the differentiation of this 

 layer form a distinct mesoblast and hypoblast laterally, while in 

 the median line they can hardly be divided into two distinct 

 layers. 



In a section shghtly further back the same is true, except that 

 we have here, in the axial line above the stellate cells, rounded ele- 

 ments derived from a forward prolongation of the cells of the 

 primitive streak. In the next section figured, passing through 



