RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 857 



views from those which have the characters just described, the 

 hypoblastic sheet is often much less completely differentiated, 

 and we have met with other blastoderms, again, in which the 

 hypoblastic sheet was completely established, except at the 

 hinder part of the embryonic shield ; where, in place of it and 

 of the cells between it and the epiblast, there was only to be 

 found a thickish layer of rounded cells, continuous behind with 

 the germinal wall. 



In the next stage, of which we have examined surface views 

 and sections, there is already a well-formed primitive streak. 



The area pellucida is still nearly spherical, the embryonic 

 shield has either disappeared or become much less obvious, but 

 there is present a dark linear streak, extending from the pos- 

 terior border of the area pellucida towards the centre, its total 

 length being about one third, or even less, of the diameter of 

 the area. This streak is the primitive streak. It enlarges con- 

 siderably behind, where it joins the germinal wall. By Koller 

 and Gerlach it is described as joining the sickle-shaped struc- 

 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 45, 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 Kupffer and Benecke 1 , 

 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 (PL 43, 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- 



1 " Photogramme d. Ontogcnie d. Vogel." Nova Acta. K. Leop. Carol, Datts- 

 chen Akad. d. Naturfor, Bd. x. 41, 1879. 



B. 55 



