858 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 



definite layer, the cells of which are not so flat as is the case 

 with the hypoblast cells of the posterior part of the blastoderm, 

 and in the older specimens of this stage they are very decidedly 

 more columnar than in the younger specimens. 



The primitive streak is however the most interesting structure 

 in the area pellucida at this stage. 



The feature which most obviously strikes the observer in 

 transverse sections through it is the fact, proved by Kolliker, that 

 it is mainly due to a proliferation of the epiblast cells along an 

 axial streak, which, roughly speaking, corresponds with the dark 

 line visible in surface views. In the youngest specimens and at 

 the front end of the primitive streak, the proliferated cells do not 

 extend laterally beyond the region of their origin, but in the 

 older specimens they have a considerable lateral extension. 



The hypoblast can, in most instances, be traced as a distinct 

 layer underneath the primitive streak, although it is usually less 

 easy to follow it in that region than elsewhere, and in some 

 cases it can hardly be distinctly separated from the superjacent 

 cells. 



The cells, undoubtedly formed by a proliferation of the epi- 

 blast, form a compact mass extending downwards towards the 

 hypoblast ; but between this mass and the hypoblast there are 

 almost always present along the whole length of the primitive 

 streak a number of cells, more or less loosely arranged, and 

 decidedly more granular than the proliferated cells. Amongst 

 these loosely arranged cells there are to be found a certain 

 number of large spherical cells filled with yolk granules. Some- 

 times these cells are entirely confined to the region of the primi- 

 tive streak, at other times they are continuous laterally with cells 

 irregularly scattered between the hypoblast and epiblast (Ser.C,2), 

 which are clearly the remnants of the undifferentiated cells of 

 the embryonic shield. The junction between these cells and 

 the cells of the primitive streak derived from the epiblast is 

 often obscure, the two sets of cells becoming partially inter- 

 mingled. The facility with which the cells we have just spoken 

 of can be recognized varies moreover greatly in different in- 

 stances. In some cases they are very obvious (Ser. c), while in 

 other cases they can only be distinguished by a careful ex- 

 amination of good sections. 



