GENERATION AND DEVELOPMENT. 253 



In the Mammalia, on the other hand, there is no large unsegmented 

 mass corresponding to the food-yelk of birds; the entire ovum undergoes 

 segmentation, and is hence termed holoblastic. 



The eggs of Fishes, Reptiles, and Birds, are meroblastic, while those of 

 Amphibia and Mammalia are holoblastic. 



Of the changes which the mammalian ovum undergoes previous to the 

 formation of the embryo, some occur while it is still in the ovary, and are 

 apparently independent of impregnation: others take place after it lias 

 reached the Fallopian tube. The knowledge we possess of these changes 

 is derived almost exclusively from observations on the ova of 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 soon 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 pellucid a. 

 It is at first exceedingly fine, and, owing to this, and to its transparency, 

 is not easily recognized: but at the lower part of the Fallopian tube it ac- 

 quires considerable thickness. 



Segmentation. The first visible result of fertilization is a slight 

 amoeboid movement in the protoplasm of the ovum: this has been observed 

 in some fish, in the frog, and in some mammals. Immediately succeeding 

 to this the process of segmentation commences, and is completed during 

 the passage of the ovum through the Fallopian tube. The 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 } f elk, 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 enclosed by the zona pellucida or vitelline mem- 

 brane (Fig. 406*). Each of these little spherules contains 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 to be the central 

 vesicle contained in each division of the yelk. Originally there was prob- 

 ably but one vesicle, situated in the centre of the entire granular mass 



