358 GENERATION. 



probably in this condition that the ovum passes from the 

 Fallopian tube into the uterus, at about the eighth day after 

 fecundation. 1 



Primitive Trace of the Enibryon. The cells formed by 

 the segmentation of the vitellus, after this process is com- 

 pleted, are arranged in the form of a membrane, the blasto- 

 dermic membrane, which is farther subdivided, as develop- 

 ment advances, into layers, which will be fully described 

 hereafter. The albuminous covering which the ovum has 

 received in the upper part of the Fallopian tube gradually 

 liquefies and penetrates the vitelline membrane, furnishing, 

 it is thought, matter for the nourishment and development 

 of the vitellus. In the Fallopian tube, indeed, the adventi- 

 tious albuminous covering of the ovum presents an analogy 

 to the albuminous coverings which the eggs of oviparous ani- 

 mals receive in the oviducts ; with the difference that this 

 albuminous matter is almost the sole source of nourishment 

 in the latter, and exists in large quantity, while, in viviparous 

 animals, the quantity is small, is generally consumed as the 

 ovum passes into the uterus, and, in the uterus, the ovum 

 forms attachments to and draws its nourishment from the 

 vascular system of the mother. 



At the period when the fecundated ovum enters the 

 uterus, it has increased in size about five times. 2 It is then 

 composed of an external covering, the vitelline membrane, 

 with a cellular membrane internal to this, the blastodermic 

 membrane, and a certain amount of liquid in its interior. 



Soon after the formation of the single blastodermic mem- 

 brane, at a certain point on its surface, there appears a 

 rounded elevation, or heap of smaller cells, forming a dis- 

 tinct spot, called the embryonic spot. As development ad- 

 vances, this spot becomes elongated and oval. It is then sur- 

 rounded by a clear, oval area, called the area pellucida, and 

 the area pellucida is itself surrounded by a zone of cells, 



1 LOXGET, Traitt de physiologic, Paris, 1869, tome Hi., p. 841. 2 Ibid. 



