PRIMITIVE TRACE OF THE EMBRYON. 399 



Most of the phenomena of segmentation have been observed in the lower orders of 

 animals ; but there can be no doubt that analogous processes take place in the human 

 ovum. In the rabbit, Weil observed, forty-five and a half hours after copulation, an 

 ovum, with sixteen segmentations, situated in the lower third of the Fallopian tube. 

 Ninety-four hours after copulation, he observed an ovum, with a delicate mosaic appear- 

 ance, presenting a small, rounded eminence on its surface. 



It is impossible to say how long the process of segmentation continues in the human 

 ovum. It is stated that it is completed in rabbits in a few days, and, in dogs, that it occu- 

 pies more than eight days. When the cells of the blastoderm are completely formed, 

 they present a polygonal appearance as they are pressed against the vitelline membrane, 

 their inner surface being rounded. The ovum then contains, within the external layer 

 of cells, a small quantity of liquid. It is probably in this condition that the ovum passes 

 from the Fallopian tube into the uterus, at about the eighth day after fecundation. 



Primitive Trace of the Embryon. The cells formed by the segmentation of the vitel- 

 lus, after this process is completed, are arranged in the form of a membrane (the blasto- 

 dermic membrane) which is farther subdivided, as development advances, into layers, 

 which will be fully described hereafter. The albuminous covering which the ovum IK.S 

 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 develop- 



FIG. 290. Prim itive trace of the embryon. (Liegeois.) 



a, primitive trace ; 5, urea pellucida; c, area obscura ; d, blastodermic cells; , villl beginning to appear on the vitel- 

 line membrane. 



ment of the vitellus. In the Fallopian tube, indeed, the adventitious albuminous cover- 

 ing of the ovum presents an analogy to the albuminous coverings which the eggs of 

 oviparous animals receive in the oviducts; with the difference that this albunii 

 matter is almost the sole source of nourishment in the latter, and exist* in lar-o quantity, 

 while, in viviparous animals, the quantity is small, is generally consumed as the 

 passes into the uterus, and, in the uterus, the ovum forms attachments to and draws i 

 nourishment from the vascular system of the mother. 



At the period when the fecundated ovum enters the uterus, it has increased in ita 

 about five times. 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 membrane, at a certain point c 



