482 THE HUMAN EMERYO. 



commencing. A distinct stomatodaoal depression, DS, is present 

 on the under surface of the head. The two halves of the heart, 

 R, have united ; and the heart, now a single tube, is already 

 twisted in a characteristic S shape. Fourteen or fifteen pairs of 

 mesoblastic somites, or protovertebrse, mt, are clearly visible from 

 the surface. 



There are as yet no traces of visceral arches or clefts, nor of 

 eyes, ears, or limbs. 



On comparing the embryos E and SR (Fig. 179), with the 

 corresponding stages in the development of the rabbit, i.e. with 

 rabbit embryos towards the end of the eighth day, before closure 

 of the neural canal has occurred at any point, there are seen to 

 be several points of difference. 



The rabbit embryo at this stage is still on the surface of the 

 vesicle, while the human embryo is already covered by a well- 

 formed amnion. In connection with this separation from the 

 surface there is a further point of difference ; the human embryo 

 is connected at its hinder end by a short thick stalk (Fig. 

 179, tz) with the wall of the vesicle; while in the rabbit the 

 tail fold is only just commencing, and the hinder end of the 

 embryo is still directly continuous with the wall of the blasto- 

 dermic vesicle. 



This stalk of connection (Fig. 179, tz), between the embryo 

 and the wall of the vesicle, arises from the under surface of the 

 hinder end of the embryo, and its relations are practically iden- 

 tical with those of the allantois of a tenth-day rabbit embryo. 

 As this stalk contains a tubular diverticulum of the hind-gut, 

 and transmits the allantoic arteries and veins (cf. Fig. 198), 

 it clearly corresponds, at any rate in great part, to the rabbit's 

 allantois, and may consequently be spoken of as the allantoic 

 stalk. 



As regards amnion and allantois, the main difference between 

 the human embryo and the rabbit may be briefly expressed by 

 saying, that both amnion and allantois develop in the human 

 embryo at an earlier stage, relatively to the embryo itself, than is 

 the case in the rabbit. The probable explanation of this preco- 

 cious development of amnion and allantois, and comparatively 

 late appearance of the embryo itself, in the human species, as 

 compared with the rabbit, will be considered further on. 



