THE TERMINAL LAYERS. 357 



more than one cell thick, the name embryonal area may be 

 given, as it is from the central portion of this that the embryo is 

 developed. 



The mode of formation of the germinal layers in the rabbit 

 has been very differently described by different observers, and 

 there are several points, even of primary importance, that are as 

 yet imperfectly understood. The following description is based 

 on the independent observations of Rauber and of Kolliker, 

 which appear to be the most exact ; but the account, though 

 consistent in itself, makes it very difficult to establish any com- 

 parison between the mode of formation of the germinal layers 

 in the rabbit and that occurring in other Vertebrates, or even in 

 other Mammals; and it seems not at all improbable that further 

 investigation may necessitate considerable modification in the 

 interpretation to be put upon the appearances described. 



The great length of time that is occupied in the process, as 

 compared with the chick or frog, for example, is remarkable, 

 and may perhaps be taken as an indication that the actual 

 mode of development is a much modified one. 



The fourth day. At the end of the fourth day, as already 

 described, the wall of the blastodermic vesicle is one cell thick, 

 except in the embryonal area, where cells of two kinds are 

 present (Fig. 140). 



The fifth day. Daring the fifth day, the cells of the outer 

 layer become thinner and larger ; they also increase in number, 

 by division, as the blastodermic vesicle grows larger. In the 

 embryonal area the cells of the outer layer have the same 

 characters as in other parts of the vesicle. 



The granular cells forming the inner layer of the embryonal 

 area, on the other hand, undergo important changes. They 

 increase in number by repeated division ; they become smaller 

 in size ; and they extend further round the interior of the 

 vesicle. But the most important change is that they become 

 arranged in two layers : — (i) an upper layer of cells with large 

 nuclei, rather wider than they are long, and closely fitted 

 together at their edges so that the outlines of the cells are 

 difficult to determine ; (ii) a lower layer of very thin flat pave- 

 ment cells, similar to those of the outer layer of the vesicle, but 



