RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 119 



The sickle-shaped structure, visible in surface veins, is stated 

 by Koller to be due to a special thickening of the germinal wall. 

 We have not found this to be a very distinctly marked structure 

 in our sections. 



In the region of the area pellucida there is placed below the 

 epiblast a more or less irregular layer of cells. This layer is 

 continuous, peripherally, with the germinal wall ; and is com- 

 posed of cells, which are distinguished both by their flattened 

 or oval shape and more granular protoplasm from the epiblast- 

 cells above, to which, moreover, they are by no means closely 

 attached. Amongst these cells a few larger cells are usually 

 present, similar to those we have already described as forming 

 an important constituent of the germinal wall. 



We have figured two sections of a blastoderm of this age 

 (Ser. A, 1 and 2) mainly to show the arrangement of these cells. 

 A large portion of them, considerably more flattened than the 

 remainder, form a continuous membrane over the whole of the 

 area pellucida, excejjt usually for a small area in front, where 

 the membrane is more or less interrupted. This layer is the 

 hypoblast {Jiy^ . The remaining cells are interposed between this 

 layer and the epiblast. In front of the embryonic shield there 

 are either comparatively few or none of these cells present (Ser. a, 

 1), but in the region of the embryonic shield they are very 

 numerous (Ser. a, 2), and are, without doubt, the main cause 

 of the opacity of this part of the area pellucida. These cells 

 may be regarded as not yet completely difi'erentiated segmen- 

 tation spheres. 



In many blastoderms, not easily distinguishable in surface 

 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 poste- 

 rior 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- 



