358 THE RABBIT. 



slightly smaller ; this lower layer extends at its margin some 

 distance beyond the edge of the upper or thicker layer. 



Three regions may, therefore, be distinguished in the wall 

 of the blastodermic vesicle of the rabbit on the fifth day. 



(i) The embryonal area is a circular patch about 0-48 mm. 

 in diameter, in which three lay era of cells are present (Fig. 141) ; 

 an upper layer, CC, of thin pavement cells ; a middle layer. E, 

 of much larger, almost cubical cells ; and a lower layer, H, of 

 thin pavement cells, very similar to those of the upper layer. 

 Each of these three layers is one cell thick ; and the three- 

 layered condition is brought about by the splitting of the inner 

 mass of cells of the fourth day (Fig. 140, CD) into two, which 

 become respectively the middle and lower layers of the fifth clay. 



(ii) Surrounding the embryonal area is a border, varying 

 in width in different specimens, in which the wall of the vesicle 

 consists of two layers, as seen at the margin of Fig. 141. These 

 two layers correspond to the upper and lower layers of the three 

 present in the embryonal area ; and the two-layered condition 

 is brought about by the lower or innermost layer, H, extending 

 beyond the margin of the embryonal area. 



(iii) All the rest of the blastodermic vesicle, at this stage 

 about four-fifths of the whole periphery, consists of a single 

 layer of cells, the outermost layer, CC, of the embryonal area 

 (cf. Fig. 140). 



With regard to the ultimate fate of these layers, it may be 

 mentioned at once that in the embryonal area, according to 

 Rauber and Kolliker, the uppermost layer of cells, CC, often 

 spoken of as Rauber's layer, disappears altogether ; the middle 

 layer, E, becomes the epiblast ; and the lower layer, H, becomes 

 the hypoblast ; so that, according to these observers, both epi- 

 blast and hypoblast are formed from the inner mass of cells of 

 the fourth day. 



This interpretation involves very considerable difficulties, 

 and will not improbably require revision. The disappearance of 

 Rauber's layer from the embryonal area, and its persistence as 

 the outer wall of the vesicle in other parts, together with the 

 derivation of both epiblast and hypoblast from the original inner 

 layer of cells, are difficult to reconcile with the course of develop- 

 ment in other Mammals; and further investigation is much 

 needed on these points. 



