CHAPTER XIX. 



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. 



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 protoplasmic 

 zona pellucida, and containing the germinal vesicle and germinal spot situ- 

 ated eccentrically. The yolk granules are of different sizes, from the 

 minutest molecules up to a diameter of T oVoth to y^nrtli of an inch 

 (about 25/j.). 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 the travelling of the germinal vesicle to the surface, and the disap- 

 pearance of its inclosing membrane, with a consequent indentation and 

 indistinctness of its outline. Its protoplasm becomes to a considerable 

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

 pears. The next step in the process is the appearance in the yolk 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, with the stars at 

 either end lying near the surface of the yolk. This spindle next becomes 

 vertical, the nucleus divides into two parts, and that nearer the surface 

 protrudes from the ovum enveloped in a, protoplasmic mass, which by 

 constriction forms the first polar cell. A second polar cell arises in the 

 same way. The remaining daughter nucleus again divides one-half of 

 it is extruded from the ovum, forming a second polar cell; the other 

 half remains behind and is called the female pro-nucleus. This is 

 clearly derived from the original germinal vesicle. It must be remem- 

 bered 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). 



Balfour's view as to the formation of the polar bodies may be given 



766 



