476 THE HUMAN EMBRYO. 



cells as hypoblastic, and as destined to become liollowed out at 

 a later stage to form the yolk-sac. 



Concerning this comparison, it must be borne in mind that 

 we have as yet no satisfactory knowledge of the histological 

 structure of these early human ova, and that the stage is one 

 about which much doubt exists even in the case of the rabbit. 

 It must further be noticed that there are some points of impor- 

 tance which tell directly against the interpretation suggested by 

 Reichert and His. 



In the first place, there is nothing in the blastodermic vesicle 

 of a rabbit on the sixth day that can be compared with the 

 central mass of cells in Eeichert's ovum. 



Secondly, if His is right in interpreting this central mass of 

 cells as the yolk-sac, then the yolk-sac of the human embryo is 

 developed in a manner entirely diflferent from that of the rabbit. 

 In the rabbit (Fig. 146), the yolk-sac is part of the blastodermic 

 vesicle itself, while in the human embryo it appears to be, from 

 the first, independent of this. In other words, if the central 

 mass of cells in Eeichert's ovum is the yolk-sac, and the later 

 stages strongly support this interpretation, then the wall of the 

 vesicle of Reichert's ovum cannot correspond to the wall of the 

 rabbit's blastodermic vesicle (c/. Figs. 146 and 188). 



Thirdly, there is strong reason for thinking, as already 

 noticed, that the wall of Reichert's ovum is double ; an inner 

 mesoblastic lining being already present, as well as the outer 

 epithelial layer. To this inner layer, if it really exists, there is 

 nothing corresponding in the rabbit's blastodermic vesicle until 

 a later stage. 



On the whole, then, the evidence, while not excluding a 

 general correspondence in grade of development between 

 Reichert's ovum and a rabbit's blastodermic vesicle of about the 

 sixth day, appears to be against a close or exact agreement 

 between the two. There are features in Reichert's ovum which 

 do not fit in with the processes of development as known in 

 the rabbit, or in other Mammals ; peculiarities which will pro- 

 bably not be understood until opportunity has occurred for 

 study of the segmentation of the human ovum, and of the stages 

 immediately following it. Light will perhaps be thrown on the 

 question by investigations on Mammals more nearly allied to 

 man than are rabbits or dogs. 



