OVUM IN THE OVIDUCT. 523 



the twelfth week after delivery, (a and b, Dr. Patterson ; d. Dr. Mont- 

 gomery.) 



II. Fertilized Ovum in the Oviduct. 



Such being a description of the ordinary or unfertilized ovum, we have 

 next to follow the changes which ensue if fertilization has taken place. 



The spermatozoa having become enveloped in the pellucid zone or 

 passing through it, the ovum is received by the iimbriated extremities 

 of the Fallopian tube, along which it is carried by peristaltic contraction 

 or ciliary motion. The first change which takes place in it is the disap- 

 pearance of its germinal vesicle and germinal spot. This disappearance 

 is, however, stated by some to be preceded by a development of cells 

 originating in the nucleus or germinal spot ; nor is it the result of fertil- 

 ization, since it occurs in the unimpregnated ovum. The Chano . es of the 

 cells of the membrana granulosa, which surround the ovum, fertilized ovum 

 become first of a conical shape, but their rounded form is re- 

 sumed on passing into the tube. 



Fig. 253. Fig. 253, ovarian ovum of dog, exhibiting the 



elongated form and stellate arrangement of the 

 cells of the discus proligerus round the zona pel- 

 lucida. 



Fig. 254, same ovum after the 

 removal of most of the club- 

 shaped cells. 



The yolk is next observed to 

 ovarian ovum. contract so as to leave a clear 



space between it and the zona pellucida. As the pas- 

 sage along the tube is taking place, the zona assumes 

 a coating of albuminous material, which is what is call- 

 ed in birds the white of the egg. It eventually becomes the chorion. 

 Meantime, after the disappearance of the germinal vesicle, a new cell, the 

 embryo cell, arises, and this undergoes subdivision or segmentation, an 

 effect in which the yolk itself presently becomes involved, each new or 

 daughter embryo cell so arising assuming a part of the yolk. A constant 

 process of bisection is thus established, the yolk dividing first into two 

 portions, then into four, eight, sixteen, etc., each division containing a 

 nucleated cell. At this period may be seen the spermatozoa involved in 

 the zona pellucida, and, as the process of bisection goes on, The mulberry 

 the mass assumes a mulberry aspect, and finally becomes mass - 

 granular. This is, for the most part, finished by the time the ovum en- 

 ters the uterus. 



Fig. 255, ova of the dog in various stages : a, from the oviduct, half 

 an inch from the uterus, spermatozoids being in the pellucid zone, the yolk 



