370 THE EABBir. 



tail end of the embryo, partly by depression of the embryo into 

 the yolk-sac, and partly by the actual uprising of a fold of the 

 somatopleure, or body wall. 



After it is once started, the amnion grows rapidly, and by 

 the end of the tenth day has spread forwards so as to roof 

 over the whole body of the embryo. In front of the embryo 

 it meets and fuses with the somatopleure, at the anterior border 

 of the pro-amnionic pit (Fig. 147). 



Apart from the prominent share taken by the tail-fold, the 

 formation and relations of the amnion are practically the same 

 in the rabbit as in the chick. As the amnion is a fold of 

 somatopleure (Figs. 146, 147), the space between its inner and 

 outer layers is necessarily continuous, as in the chick, with the 

 coelom or body cavity of the embryo. 



4. The Allantois. 



The allantois arises, on the ninth day (Fig. 146, ta), as a 

 hollow diverticulum from the ventral surface of the hinder end 

 of the alimentary canal, appearing almost like a posterior pro- 

 longation of the embryo itself. It consists, at first, of a thick 

 wall of mesoblast, in which the allantoic vessels develop very 

 early, and a lining of hypoblastic epithelium ; and from its first 

 appearance it is in very close relation with the amnion, or 

 actually continuous with this (Fig. 146). 



As the amnion extends forwards, the allantois grows with 

 it, spreading rapidly between its two layers, and over the back 

 of the embryo (Fig. 147, 148, ta). Owing to its early fusion 

 with the outer layer of the amnion, the upper surface of the 

 allantois lies practically in contact with the wall of the uterus. 



The cavity of the allantois is at first small, but from the 

 tenth to the twelfth days it enlarges v6vj greatly (Figs. 147, 

 148, ta). 



5. The Placenta. 



The placenta is the organ through which the embryo receives, 

 from the blood of the mother, the nutriment by which it is enabled 

 to develop. It is a structure of great importance and great 

 complexity ; the mode of its formation will be dealt with fully 

 at the end of this chapter, but a brief outline may be given here, 

 in order to render its relations to the blood-vessels and other 

 organs of the embryo more intelligible. 



