THE aEAAFIAN FOyLICLES. 349 



Having reached its full size, the Graafian follicle ruptures at 

 its most prominent part, and the ovum is discharged on the 

 surface of the ovary, from which it is normally taken up at once 

 by the fimbriated mouth of the oviduct. After discharge of the 

 ovum, the walls of the Graafian follicle undergo a series of curious 

 changes, which will be more fully described in the chapter deal- 

 ing with the human embryo, and which result in the formation 

 of the corpus luteum, a body which disappears early if the ovum 

 is not fertilised ; but which, if the ovum is fertilised and develops 

 into an embryo, persists in the ovary during the whole period of 

 development, and is even recognisable at the time of birth of the 

 young rabbit. 



Of the two layers of the Graafian follicle, the outer (Pig. 133, 

 Gb) is sometimes spoken of as the tunica granulosa ; and the 

 inner, GO, as the discus proUgerus ; these names, however, and 

 especially the latter one, are badly chosen, and it will be well if 

 they drop out of use altogether. 



The ovum, surrounded by the inner layer of the Graafian 

 follicle, may be attached to any part of the outer layer of the 

 follicle : it not uncommonly lies at the side nearest the surface 

 of the ovary, but it may occur at the opposite or deepest part of 

 the follicle, or at any other part of its inner surface. 



The meaning of the Graafian follicle has been much debated : 

 the most probable explanation seems to be that it is in some 

 way associated with the great diminution in size which there is 

 strong reason for thinking that the ovum has undergone during 

 the evolution of the existing types of Mammals. 



3. Maturation of the Egg. 



The nucleus of the ovum is at first centrally placed : but 

 some time before the Graafian follicle reaches its full development, 

 the nucleus moves towards the surface of the ovum. The exact 

 changes that then occur have not been determined with certainty 

 in the case of the rabbit : so far as they are known, they agree 

 closely with those already described in the case of the frog. 



A thin, homogeneous vitelline membrane is formed within 

 the zona radiata, and apparently from the egg itself : the nucleus 

 of the egg becomes inconspicuous; the yolk retracts slightly 

 from the vitelline membrane, and the first polar body is extruded 

 from the egg. 



