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HANDBOOK OF PHYSIOLOGY. 



Of the changes which the mammalian ovum undergoes previous to 

 the formation of the embryo, those which occur while it is still in the 

 ovary are independent of impregnation: others take place after it has 

 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. 



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 sub- 

 stance, 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. 



Segmentation. The first visible re- 

 sult 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. Im- 

 mediately succeeding to this the process 

 of segmentation commences, and is com- 

 pleted during the passage of the ovum 

 through the Fallopian tube. In mam- 

 mals, in which the process is an example 

 of complete segmentation, the yelk becomes 

 constricted in the middle, and surrounded 

 by a furrow which gradually deepening, 

 at length cuts it 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 vUdline spheres, the 

 whole still inclosed by the zona pellucida 

 or vitelline membrane (Fig. 442). Each of these little spherules con- 

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



