864 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 



sheet of stellate cells is continuous laterally with cubical hypo- 

 blast cells. 



As the primitive streak is approached an axial prolongation 

 forwards of the rounded and closely-packed mesoblastic ele- 

 ments 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 difficult to say which mesoblast cells are directly 

 derived from the primitive layer of hypoblast in front of the 

 primitive 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. 45, Ser.l, I, 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 slightly further back the same is true, except 

 that we have here, in the axial line above the stellate cells, 

 rounded elements derived from a forward prolongation of the 

 cells of the primitive streak. In the next section figured, pass- 

 ing through the front end of the primitive streak, the axial cells 

 have become continuous with the axial mesoblast of the primi- 

 tive streak, while below there is an independent sheet of flattened 

 hypoblast cells. 



The general result of our observations on the part of the 

 blastoderm in front of the primitive streak during this stage is 

 to shew that the primitive hypoblast of this region undergoes 

 considerable changes, including a multiplication of its cells; and 

 that these changes result in its becoming differentiated on each 

 side of the middle line, with more or less distinctness, into (i) a 

 hypoblastic sheet below, formed of a single row of flattened cells, 

 and (2) a mesoblast plate above formed of stellate cells, while in 

 the middle line there is a strip of stellate cells in which there is 

 no distinct differentiation into two layers. 



