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HANDBOOK OF PHYSIOLOGY. 



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 obser- 

 vations 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 result 

 of fertilization is a slight amoeboid move- 

 ment 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 segmen- 

 tation commences, and is completed dur- 

 ing the passage of the ovum through the 

 Fallopian tube. In mammals, in which 

 the process is an example of complete seg- 

 mentation, the yolk becomes constricted 

 in the middle, and is surrounded by a 

 furrow which, gradually deepening, at 

 length cuts it in half, while the same pro- 

 cess begins almost immediately in each 

 half of the yolk, 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 yolk 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 (fig. 460). Each of these lit- 

 tle spherules contains a transparent vesicle, like an oil-globule, which is 

 seen with difficulty, on account of its being enveloped by the yolk-gran- 

 ules which adhere closely to its surface. 



Fig. 460. Diagrams of the vari- 

 ous stages of cleavage of the yolk. 

 (Dal ton.) 



