120 y. M. BALFOUR AND F. DEIGHTON. 



ture already spoken of. We have in some instances found the 

 posterior end of the primitive streak extending laterally in the 

 form of two wings (PL IX^ fig. l). These extensions are, no 

 doubt, the sickle ; but the figures given by Koller appear to us 

 somewhat diagrammatic. One or two of the figures of early 

 primitive streaks in the sparrow, given by Kupfi'er and Benecke,^ 

 correspond more closely with what we have found, except that 

 in these figures the primitive streak does not reach the end of 

 the area pellucida, which it certainly usually does at this early 

 stage in the chick. 



Sections through the area pellucida (PI. Yll, ser. b and c) 

 give the following results as to the structure of its constituent 

 parts. 



The epiblast cells have undergone division to a considerable 

 extent, and in the middle part_, especially, are decidedly more 

 columnar than at an earlier stage, and distinctly divided into 

 two rows, the nuclei of which form two more or less distinct 

 layers. 



In the region in front of the primitive streak the cells of the 

 lower part of the blastoderm have arranged themselves as a de- 

 finite 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 



^ " Photogramme d. Ontogenie d. Vogel." Nova Acta. K. Leop. Carol, 

 ' Deutscheu Akad. d. Naturfor.,' Bd. x, 41, 1879. 



