CHAPTER XXIII. 



DEVELOPMENT. 



CHANGES WHICH OCCUR IN THE OVUM. 



OF the changes which take place in the ovum, some occur before and 

 are as it were preparatory to impregnation, and others ensue after 

 impregnation. It will be as well to consider the respective changes 

 separately. 



(1.) Changes prior to Impregnation. These changes especially 

 concern the germinal vesicle, and have been observed chiefly in the ova of 

 low types. The ovum when ripe and detached from the ovary consists, 

 it will be remembered, of a granular yolk inclosed within the protoplas- 

 mic zona pellucida, and containing the germinal vesicle and germinal 

 spot situated eccentrically. The yelk granules are of different sizes, 

 from the minutest molecules up to a diameter of -p^th to TTVo^h of an 

 inch. The germinal vesicle consists of reticulated protoplasm inclosed 

 in a distinct membrane, and containing one or more nucleoli or germinal 

 spots. The primary change observed in the ovum consists in alterations 

 in the shape of the vesicle, the disappearance of its protoplasmic reticu- 

 lurn, and of its inclosing membrane, with a consequent indentation and 

 indistinctness of its outline. Its protoplasm becomes to a considerable 

 extent confounded with the yelk substance, and its germinal spot disap- 

 pears. The next step in the process is the appearance in the yelk of two 

 stars in a clear space near the poles of the vesicle elongated to a certain 

 extent, and from this results a nuclear spindle, corresponding to a nucleus 

 in the process of division, with the stars at either end lying near the 

 surface of the yelk. This spindle next becomes vertical, and the star 

 nearer the surface protrudes from the ovum enveloped in a protoplasmic 

 mass, which by constriction form the first polar cell. A second polar 

 cell arises in the same way. From the remainder of the spindle within 

 the yelk two or three vesicles arise, and by the junction of these a single 

 nucleus is formed, which is called the female pro-nucleus. This is clearly 

 derived from the original germinal vesicle. It must be remembered that 

 these changes have been so far observed only in a certain number of 

 instances. It is very possible, not to say probable, that such changes 

 are universal in the animal kingdom (Balfour). 



