476 THE OVARIES. 



peculiar opaque granular and prismatic cells in several layers, forming 

 the membrana or tunica granulosa of authors. In the earliest stages of 

 formation, before the follicle has expanded, this layer of cells lies 

 close to the ovum, but as the follicles enlarge tliere comes to be formed 

 within the granular layer a space of variable size filled with clear fluid, 

 which holds a quantity of alb-amen in solution, so as to be coagulable by 

 heat and chemical re-agents. The ovum is now found to be embedded 

 in a thickened portion of this cellular layer, the cumulus or discus pro- 

 lif/crus of Von Baer, so as to be placed near the inner surface on one side 

 of the follicle. The position of the ovum within the follicle appears to 

 be subject to some variation. It was formerly described as being always 

 on the side next the surface of the ovary ; but according to more recent 

 observations it is as frequently situated on the deeper side of the 

 follicle. The cells of the tunica granulosa and discus proligerus are 

 without distinct external walls ; they possess a considerable quantity 

 of protoplasm of an opaque or granular aspect, and are provided with 

 oval nuclei. Near the inner surface of the follicle, and on the outer 

 surface of the ovum, they assume a more elongated prismatic or pedi- 

 culated form than in other parts of the membrane. 



The ovarian ovum. — Each Graafian follicle usually contains only a 

 single ovum, but occasionally, though seldom, two ova, and very rarely 

 three, have been observed in the same follicle. TJie ovum, first dis- 

 covered by Von Baer in 1827, is a spherical body of about -^^^ of an 

 inch in diameter, aud of an opaque yellowish white colour. When 

 fully formed it consists essentially of the following parts, viz.: 1st., a 

 firm transparent external vesicular membrane termed zona j)eUuckIa, or 

 vitelline membrane; 2nd., a mass of granular protoplasm filling the 

 vesicle and constituting the yelh, yolk or riteUus, in which a number of 

 oil globules of variable size are suspended ; ord, embedded in the yolk 

 protoplasm, and situated on one side near the surface, a small clear 

 vesicle of about y^o of an inch in diameter, filled with a finely granular 

 fluid, the f/enninal vesicle ; and 4th, within this vesicle a dark granular 

 spot, of about 3-0^ of an inch in diameter, the macula gcrminativa. 



Fig. 337. Fig. 337. — Ovum of the Sow kemoyed feoii the Graafian 



Follicle, viitn iis cellular covekixq (from M. Larry). 



1, germinal spot ; 2, germinal vesicle ; 3, yolk ; 4, zone 

 pellucida or external covering of the ovum ; 5, part of 

 the tunica granulosa or proligerous disc ; G, some adherent 

 cells. 



The wnole ovum was compared by Schwann to 



an organic cell, and this view is still in general 



maintained ; the zona representing the cell wall, 



the yolk the protoplasmic cell contents, or then* 



product, the germinal vesicle the nucleus, and the macula the nucleolus. 



The part of the ovum which is first formed is the germinal vesicle, 



or nucleus, which, at so early a period as the tenth or eleventh week of 



foetal life, has been observed to take its origin from one of the cells 



of the epithelial covering or germinal epiihelium of the incipient ovary, 



by its undergoing enlargement and becoming embedded in the ovarian 



substance. The inclosing ovarian stroma thus constitutes the first 



form of a Graafian follicle, and follicle and germ may be considered 



