856 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 



Below the epiblast there is placed, in the peripheral part of 

 the area opaca, simply white yolk ; while in a ring immediately 

 outside and concentric with the area pellucida, there is a closely- 

 packed layer of cells, known as the germinal wall. The con- 

 stituent cells of this wall are in part relatively small, of a 

 spherical shape, with a distinct nucleus, and a granular and not 

 very abundant protoplasm ; and in part large and spherical, 

 filled up with highly refracting yolk particles of variable size, 

 which usually render the nucleus (which is probably present) 

 invisible (A, I and 2). This mass of cell rests, on its outer side, 

 on a layer of white yolk. 



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, i and 2) mainly to shew 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, except 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, i), 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 differentiated seg- 

 mentation spheres. 



In many blastoderms, not easily distinguishable in surface 



