ascertained beyond doubt, that segmentation of the yelk is the result of impregnation alone, and that it never takes place in the unim pregnated ovum.
This segmentation of the yelk consists in a spontaneous cleavage of that body, at first into two, and then into four, equal parts; the process of division continuing in geometric progression until the whole is broken up into a mass of finely nucleated particles, between which the original sperm-force is probably equally divided.
Segmentation of the yelk of the mamma lian ovum has never been observed in its cornmencing stages anywhere but in the tube. The extent to which .it proceeds betbre the ovum quits the oviduct to enter the uterus appears to vary in different species. Bischoff never saw more than four yelk-divisions in the ovum of the Guinea-pig by the time that This change is preliminary to another oc currence, which has been observed in the ova of many animals, both vertebrate and invertebrate, viz. the rotation of the yelk within the interspace just described ;— a ro tation which is effected by the aid of cilia clothing the surface of the yelk.
About this time may be observed one, or perhaps two, small granular bodies, whose formation has given rise to many and varied speculations regarding their signification and use. They occupy a portion of the space between the yelk and zona pellucida, and appear to be common to the mammalian ovum and that of other classes. The most probable supposition regarding their use con nects them with the division or cleavage of the yelk which follovvs their appearance.
Whatever doubts may be entertained as to the dependence of the phenomena already described upon a preceding act of impregna tion, all question is set at rest at this point, by the direct experiments of Newport, who it had reached the lower portion of the tube ; and it is probable that a further division into eight parts occurs in the extreme end of the duct, since, in the next condition of the ova found in the uterus, the yelk exhibited 12 —16 divisions.
The only remaining change in the condition of the ovum during its residence in the ovi duct, which it is necessary here to notice, is the addition, sometimes, of a thick layer of albumen around the zona pellucida, which is formed upon it in the middle and lower por tions of the tube. But,this is certainly not a constant, and apparently not even a common occurrence. It occurs in the rabbiti but not
in the bitch, Guinea-pig, or roe.
These are the principal and more obvious changes which the ovum experiences in its passage down the Fallopian tube until it enters the uterus. So regular is the order with which they succeed each other that particular portions of the tube may be as signed as the seat of each occurrence. Thus the first, or upper third of the oviduct is appropriated to the reception of the ovum, which, soon after quitting the ovary-, is here deprived of its adventitious covering of nu cleated cells, and is thus prepared for the full operation of the spermatozoa, whose active movements in this part of the tube have been frequently noticed. Here also spermatozoa have been frequently seen upon, and even within, the ova ; and here the first changes characteristic of the commencing operations of the sperm force, such as the formation of the respiratory chamber, and rotation of the yelk, may be noticed. In the middle of the tube the ova commonly- exhibit still more decided evidences of impregnation. The cleavage of the yelk has already com menced, and one or more granular bodies occupy the space between it and the zona. The ova found in the lower third, except those which may be destined to• perish, al ways show unmistakable signs of impregna tion, of which the segmentation of the yelk, now advanced to the production of 12-16 divisions, is the most expressive.
If the views of Bischoff' be correct, it is in the upper third, or at farthest in the middle of the tube, that impregnation rnust occur, unless indeed it takes place at the ovary. For in the lower end of the tube the more definite developmental changes of the ovum occur, or otherwise the ovum perishes. In the dog and Guinea-pig, by the time the ovum has reached this spot, the cestrus is past, and the animal will no longer permit the coitus.* Connected apparently with some of the foregoing steps in the process of generation, though it does not appear precisely with which, is a phenomenon described by Bis choff as occurring in the Guinea-pig. Several times Bischoff had the good fortune to ob serve with a lens, and also under the micro scope, a peristaltic action in the walls of the oviduct, by which the contained ova, visible through them, were moved backwards and for wards. The ova appeared to be surrounded by a transparent fluid, in which they floated.