III.] THE GERMINAL WALL. 65 



side are situated the mesoblastic plates (B). The hypo- 

 blast forms a continuous and nearly flat layer below. 



While the changes just described have been occur- 

 ring in the area pellucida, the growth of the area opaca 

 has also progressed actively. The epiblast has greatly 

 extended itself, and important changes have taken place 

 in the constitution of the germinal wall already spoken 

 of. 



The mesoblast and hypoblast of the area opaca do 

 not arise by simple extension of the corresponding layers 

 of the area pellucida ; but the whole of the hypoblast 

 of the area opaca, and a large portion of the meso- 

 blast, and possibly even some of the epiblast, take their 

 origin from the peculiar material which forms the 

 germinal wall and which is continuous with the hypo- 

 blast at the edge of the area opaca (vide figs. 15, 17, 

 18, 19, 20). 



The exact nature of this material has been the subject of 

 many controversies. Into these controversies it is not our purpose 

 to enter, but subjoined are the results of our 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 appears to 

 extend itself over the yolk, partly at the expense of the cells of 

 the germinal wall, and possibly even of cells formed around the 

 nuclei in this part. The cells of the germinal wall, which are at 

 first well separated from the yolk below, become gradually ab- 

 sorbed in the growth of the hypoblast, and the remaining cells 

 and yolk then become mingled together, and constitute a com- 

 pound structure, continuous at its inner border with the hypo- 

 blast. 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 rather large cells imbedded 

 F. & B. 5 



