EARLY STAGES OF DEVELOPMENT OF THE RABBIT. 155 



have drawn a figure (fig. 36, PL 17) from a specimen aged six 

 days five hours. This is very similar to KoUiker's aged six 

 days. It is a silver nitrate specimen. This was drawn when 

 the extreme upper surface was in focus. On focussing down it 

 is possible to find small cell outlines under the large areas, but 

 finer than those of the cells {EP. /.). On focussing still further 

 down, the outlines of the hypoblast layer are visible as very 

 fine, very wavy lines, forming as a rule more regular areas. 

 The small cells {EP. I.) are present throughout the embryonic 

 disc, but in some places are much more marked than others ; 

 that is to say, in some places the silver nitrate produces the 

 characteristic black marks between the cells more strongly 

 than in others. For this there must be a reason, and this 

 seems to be that in some parts the small cells are at the 

 surface (EP: I.) ; at others {EP. O.) they are covered by some 

 other body, this other body being certain cells of the outer 

 layer of epiblast (Rauber cells). 



This is an intermediate stage. Earlier the outer layer is 

 quite continuous over the whole of the embryonic disc, as 

 shown in KoUiker's figs. 11 and 12, pi. ii (see also my sections 

 of these stages, figs. 27, 28, and 30). In these the outlines of 

 the outer layer are distinct as large polygonal areas. In the 

 earlier* stages the outlines of the inner layer of epiblast are 

 not sharply defined in silver nitrate specimens, apparently 

 because they are but loosely arranged. But in KoUiker's figs. 

 11 and 12, which represent specimens at the latter end of the 

 sixth or beginning of the seventh day, the small cells, i. e. the 

 inner layer of epiblast which now forms a more compact layer, 

 show outlines which are visible below the outer layer cells, 

 although faint. 



I entirely agree with Kolliker that this outer layer (Rauber's 

 layer) now becomes broken and, as it were, torn up into 

 isolated cells or little patches of cells, and I think that the 

 cause of this may be as follows. I have above given reasons 

 for supposing that the cells of the embryonic disc region are 

 comparatively inactive during the fifth day and first part of 

 the sixth day. The outer layer of cells (Rauber's layer), I 



