AVES. 133 



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 

 Hist 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 tigs. 98 and 100, n) is usually present at the 

 junction of the hypoblast with the germinal wall. 



Ths 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 con- 

 tain 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 Kulliker (No. 135) 

 and Virchow (No. 150) : they contain however nothing which can be con- 

 sidered as a nucleus. Between them however nuclei 1 may easily be seen 

 in specimens hardened in picric acid, and stained with hjematoxylin (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 

 granule r protoplasm, which is continuous with a mesh work of granular 

 protoplasm enveloping the above described vesicles. The germinal wall 

 is still continuous with the hypoblast at its edge; and close to the 

 junction of the two the hypoblast at first forms a layer of moderately 

 columnar cells, one or two deep and directly continuous with the ger- 

 minal wall, and at a later period usually consists of a mass of rounder 

 cells lying above the somewhat abrupt inner edge of the germinal wall. 



The germinal wall certainly gives rise to the hypoblast cells, which 

 mainly grow at its expense. They arise at the edge of the area pellucidu, 

 and when first formed are markedly columnar, and enclose in their proto- 

 plasm one of the smaller vesicles of the germinal wall. 



In the later stages (fourth day and onwards) the whole germinal wall 

 is stated to break np into columnar hypoblast cells, each of them mainly 

 formed of one of the vesicles just spoken of. After the commencing 

 formation of the embryo the mesoblast becomes established at the inner 

 edge of the area opaca, between the germinal wall and the epiblast ; and 



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 interpre- 

 tations. 



