160 GERMINAL WALL. 



the hypoblast at the edge of the area opaca (vide figs. 91, 94, 97, 

 98, 99, 100). 



The exact nature of this material has been the subject of many contro- 

 versies. Into these controversies it is not my purpose to enter, but sub- 

 joined are the results of my own examination. The germinal wall first 

 consists, as already mentioned, of the lower cells of the thickened edge of 

 the blastoderm, and of the subjacent yolk material with nuclei. During the 

 period before the formation of the primitive streak the epiblast extends 

 itself over the yolk, partly, it appears, at the expense of the cells of the 

 germinal wall, and possibly even of cells formed around the nuclei in this 

 part. This mode of growth of the epiblast is very similar to that in the 

 epibolic gastrulas of many Invertebrata, of the Lamprey, etc. ; but how far 

 this process is continued in the subsequent extension of the epiblast I am 

 unable to say. The cells of the germinal wall, which are at first well 

 separated from the yolk below, become gradually absorbed in the growth of 

 the hypoblast, and the remaining cells and yolk then become mingled 

 together, and constitute a compound structure, continuous at its inner 

 border with the hypoblast. This structure is the germinal wall usually so 

 described. It is mainly formed of yolk granules with numerous nuclei, and 

 a somewhat variable number of largish cells imbedded amongst them. The 

 nuclei typically form a special layer immediately below the epiblast, some of 

 which are probably enclosed by a definite cell-body. A special mass of nuclei 

 (vide figs. 98 and 100, 11) is usually present at the junction of the hypoblast 

 with the germinal wall. 



The germinal wall at this stage corresponds in many respects with 

 the granular material, forming a ring below the edge of the blastoderm in 

 Teleostei. 



It retains the characters above enumerated till near the close of the first 

 day of incubation, i.e. till several mesoblastic somites have become 

 established. It then becomes more distinctly separated from the subjacent 

 yolk, and its component parts change very considerably in character. The 

 whole wall becomes much less granular. It is then mainly formed of large 

 vesicles, which often assume a palisade-like arrangement, and contain 

 granular balls, spherules of white yolk, and in an early stage a good deal of 

 granular matter (vide fig. 115). These bodies have some resemblance to 

 cells, and have been regarded as such by Kolliker (No. 135) and Virchow 

 (No. 150) : they contain however nothing which can be considered as a 

 nucleus. Between them however nuclei 1 may easily be seen in specimens 

 hardened in picric acid, and stained with haematoxylin (these nuclei are not 

 shewn in fig. 115). These nuclei are about the same size as those of the 

 hypoblast cells, and are surrounded by a thin layer of granular protoplasm, 



1 The presence of numerous nuclei in the germinal wall was, I believe, first 

 clearly proved by His (No. 132). I cannot however accept the greater number of his 

 interpretations. 



