CHANGES IN THE FECUNDATED OVUM. 801 



cells. The ovum is probably in this condition when it passes from the 

 Fallopian tube into the uterus. 



Most of the phenomena of segmentation have been observed in the 

 lower forms of animals ; but there can be no doubt that analogous pro- 

 cesses take place in the human ovum. In the rabbit, forty-five and a half 

 hours after copulation, Weil observed an ovum with sixteen segmentations, 

 situated in the lower third of the Fallopian tube. He observed an ovum, 

 ninety-four hours after copulation, with a delicate mosaic appearance, pre- 

 senting 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 

 occupies more than eight days (Hermann). 



After segmentation has been completed, a cavity filled with liquid 

 appears between the hypoblastic and epiblastic cells, except at that portion 

 which has last been covered by the epiblast. Here the cells of the hypo- 

 blast are in contact with the epiblast. The liquid in the interior of the 

 ovum gradually increases in quantity, the ovum becomes enlarged to the 

 diameter of -^ to -fa of an inch (O5 to 1 mm.), and is now called the 

 blastodermic vesicle. The epiblastic cells surround the blastodermic ves- 

 icle completely, forming a single layer over the greater portion ; and the 

 hypoblastic cells form a lenticular mass attached to the smaller portion 

 of the inner surface of the layer of epiblastic cells (see Fig. 293, c and D). 

 It is at this portion of the ovum that the embryonic spot or area afterward 

 appears. 



The albuminous covering which the ovum has received in the upper 

 part of the Fallopian tube gradually liquefies and penetrates the vitel- 

 line membrane, furnishing, it is thought, matter for the nourishment 

 and development of the vitellus. In the Fallopian tube, indeed, the 

 adventitious albuminous covering 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 albuminous matter is almost 

 the sole source of nourishment in the latter and exists in large quan- 

 tity, while in viviparous animals the quantity is small, is generally con- 

 sumed 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. 



Primitive Trace. Soon after the formation of the blastodermic vesicle, 

 at a certain point on its surface there appears a rounded elevation or heap 

 of smaller cells, forming a distinct spot, called the embryonic spot. As 

 development advances, this spot becomes elongated and oval. It is then 

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

 pellucida is itself surrounded by a zone of cells, more granular and darker 

 than the rest of the blastoderm, called the area opaca. The line thus 

 formed and surrounded by the area pellucida is called the primitive trace. 

 This primitive trace, or primitive groove, however, is a temporary structure. 

 After the groove is formed, there appears, in front of but not continuous 



