CHANGES IN THE FECUNDATED OVUM. 



801 



the polar globule, and there is then a furrow on the opposite side, both deep- 

 ening until the entire vitellus is divided into two globes. These are at first 

 spherical ; but they soon become flattened upon each other, into two hemi- 

 spheres. There follows then a similar division into four, another into eight, 

 and so on, until the entire vitellus is divided into small cells. It is probable 

 that at first the cells of the vitellus have no membrane ; but a membrane is 

 soon formed, a nucleus and a nucleolus appear, and the cells are perfect. 

 The ovum is then called the morula. The cells measure y-gVg- to ysVir of 

 an inch (20 to 25 p.] in diameter. 



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

 forms of animals ; but there can be no doubt that analogous processes 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, 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 com- 

 pleted in rabbits in a few days, and in dogs, that it occupies more than eight 

 days (Hermann). 



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 certain quantity of liquid, and is increased in size to 

 the diameter of -fa to ^ of an inch (0*5 to 1 mm.). 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. The 

 cells formed by the segmen- 

 tation of the vitellus, after 

 this process is completed, 

 are arranged in the form of 

 a membrane (the blastoder- 

 mic membrane) which is 

 farther subdivided, as de- 

 velopment advances, into 

 layers, which will be de- 

 scribed hereafter. The al- 

 buminous covering which 

 the ovum has received in 

 the upper part of the Fal- 

 lopian tube gradually liquefies and penetrates the vitelline membrane, furnish- 

 ing, it is thought, matter for the nourishment and development of the vitel- 

 lus. In the Fallopian tube, indeed, the adventitious albuminous covering of 



FIG. 294. Primitive trace of the embryon (Ligeois). 

 a, primitive trace; 6, area pellucida; c, area opaca; d, blasto- 

 dermic cells ; e, e, villi beginning to appear on the vitelline 

 membrane. 



