MATURATION OF OVUM 



11 



through the protoplasm as a sort of framework. These again disappear as the 

 deutoplasm collects in the centre of the cell in the form of minute vacuoles with 

 clear contents, fat-spheres, and minute highly refractile bodies (fig. 19). 



When growth and storage of yolk-food are completed, the ovum is matured 

 by the extrusion of the polar bodies. The maturation stages now to be 

 described have not been seen in the human ovum, but observations on other 

 mammals put it beyond question that the process occurs in essentially the same 

 way as in lower forms. The phases leading up to maturation have been described 

 by Heape l in the rabbit. 



Prior to the beginning of the sexual season in that animal certain of the 

 Graafian vesicles enlarge, and the growth of those near the surface causes them 

 to project and form swellings on the surface of the ovary. The wall of the follicle 

 and the tunic of the ovary is here much attenuated : so much so that in some of 

 them, when ripe, the structure is sufficiently transparent to allow of the ovum 



FIG. 18. EARLY STAGE OF 



The follicular epithelium has become many-layered, and a cavity has appeared among the follicular 

 cells containing the liquor folliculi. The mass of cells which encloses the ovum is the discus proligerus. 



being seen within the vesicle. During procestrum (period preceding ' heat ') the 

 blood-vessels surrounding the follicle enlarge, and these, running in the thin wall 

 which projects from the surface of the ovary, give the follicle the bright pink colour 

 which is characteristic of it. When maturation sets in, the cells of the discus 

 proligerus begin to withdraw from the ovum, and eventually remain attached to 

 the zona only by very fine protoplasmic strands. At the same time the ovum 

 withdraws from the zona, and a narrow perivitelline space appears. 



The polar bodies are now thrown off (figs. 20 and 21). They are minute portions 

 of the egg-substance budded off in quick succession from the same point on the 

 surface into the perivitelline space. In this they persist for a time, but ultimately 

 disappear. They were originally termed polar bodies or directive corpuscles, from 

 the supposition that their presence determines the pole of the egg at which the 

 first segmentation will take place should the ovum become fertilised. As a matter 

 of fact, they occupy the pole which becomes afterwards the vegetative pole (Van 



1 Proc. Roy. Soc. B. vol. Ixxvi. 1905. 



