CLEAVAGE OF THE YELK. 579 



mals, especially the bitch and rabbit : but it may be inferred 

 that analogous changes ensue in the human ovum. 



Bischoff describes the yelk of an ovarian ovum after coitus 

 as being unchanged in its characters, with the single exception 

 of being fuller and more dense ; it is still granular, as before, 

 and does not possess any of the cells subsequently found in it. 

 The germinal vesicle always disappears, sometimes before the 

 ovum leaves the ovary, at other times not until it has entered 

 the Fallopian tube ; but always before the commencement of 

 the metamorphosis of the yelk. 



As the ovum approaches the middle of the Fallopian tube, it 

 begins to receive a new investment, consisting of a layer of 

 transparent albuminous or glutinous substance, which forms 

 upon the exterior of the zona pellucida. It is at first exceed- 

 ingly fine, and, owing to this, and to its transparency, is not 

 easily recognized : but at the lower part of the Fallopian tube 

 it acquires considerable thickness. 



About this time, that is to say, during its passage through 

 the Fallopian tube, a very remarkable change takes place in 

 the interior of the ovum. The whole yelk becomes constricted 

 in the middle, and surrounded by a furrow, which, gradually 

 deepening, at length cuts the yelk in half, while the same 

 process begins almost immediately in each half of the yelk, 

 and cuts it also in two. The same process is repeated in each 

 of the quarters, and so on, until at last by continual cleavings 

 the whole yelk is changed into a mulberry-like mass of small 

 and more or less rounded bodies, sometimes called "vitelline 

 spheres," the whole still inclosed by the zona pellucida or vitel- 

 line membrane (Fig. 215). Each of these little spherules con- 

 tains a transparent vesicle, like an oil-globule, which is seen 

 with difficulty, on account of its being enveloped by the yelk- 

 granules which adhere closely to its surface. 



The cause of this singular subdivision of the yelk is quite 

 obscure : though the immediate agent in its production seems 

 so be the central vesicle contained in each division of the yelk. 

 Originally there was probably but one vesicle, situated in the 

 centre of the entire granular mass of the yelk, and probably 

 derived from the germinal vesicle. This, by some process of 

 multiplication, divides and subdivides : then each division and 

 subdivision attracts around itself, as a centre, a certain portion 

 of the substance of the yelk. 



About the time at which the manimiferous ovum reaches 

 the uterus, the process of division and subdivision of the yelk 

 appears to have ceased, its substance having been resolved 

 into its ultimate and smallest divisions, while its surface pre- 



