[82] 



OVUM. 



but in some animals, in which the vesicles are 

 proportionately more expanded, they extend 

 beyond the general line of the surface in 

 various places, and even sometimes give the 

 ovary somewhat of the mammillated or grape- 

 like appearance more common among ovipa- 

 rous animals. 



The follicle is filled to distension with a 

 clear albuminous fluid, which escapes with 

 force when an incision is made through the 



Fig. 55*. 



Ovarian ovum of the Dog. (From Bischaff'.*) 



a, magnified representation of the ovarian ovum 

 of the dog, nearly mature, situated in the discus 

 proligerus and part of the cells of the granular 

 membrane; b, several detached granular cells. 



d, another ovarian ovum of the same animal 

 perfectly ripe, immediately previous to the rupture 

 of the. Graafian follicle ; the cells of the proligerous 

 disc have become of a pediculated shape ; c, some 

 of these cells detached. 



e, the ovum from the same specimen as in fig. a, 

 freed artificially from the granular cells of the 

 disc, showing externally the thick clear zona or 

 external membrane, and internally the opaque 

 yolk substance: in the latter the germinal vesicle 

 is obscured by the opacity of the substance sur- 

 rounding it. 



./*, the same ovum burst by pressure, showing 



the contents of the ovum which have escaped, 



viz. the finely granular yolk substance and the 

 germinal vesicle with its macula. 



membrane. Close to the inner surface of the 

 follicle, and surrounding the fluid, is situated 

 the layer of nucleated granular cells which 

 has been termed tunica or membrana gramilosa, 

 from the opaque granular appearance of the 

 cells composing it. These cells form a com- 

 plete vesicular lining of the follicle; but 

 throughout the greater part they cohere with 

 only a moderate degree of firmness, so that the 

 membrane readily tears when the follicle is 

 opened. The minute ovum is imbedded in a 

 thicker portion of this layer, the cumulus or dis- 

 cus proligerus of Von Baer, and is almost in- 

 variably situated close to the inner surface of 

 the most projecting part of the Graafian follicle 

 (see Jig. 54*. A. and c,, mg.} where in some 

 animals, but not in the human ovary, the 

 ovum may be detected in the undissected fol- 

 licle through its coats and the ovarian cover- 

 ings. 



The cells of the membrana gramilosa are in 

 general about -g^Vo" in diameter. They adhere 

 with considerable firmness to the surface of 

 the zona ; so that when the follicle is opened 

 and its contents are allowed to run upon a 

 plate of glass for examination, the ovum is 

 always found placed in its disc in a circum- 

 scribed attached portion of the membrana 

 granulosa of about twice its own diameter. 

 The ovum is itself of a nearly perfect sphe- 

 rical form when freed from pressure ; but 

 as it lies thus imbedded in the membrana 

 granulosa, and moistened on a plate of glass, 

 it gives rise to a slight rounded elevation in 

 that membrane, which may easily be detected 

 with the naked eye when the specimen is 

 viewed sideways. 



When the contents of the Graafian follicle 

 are discharged naturally during life, a small 

 aperture occurs nearly in the centre of the 

 most projecting part of the wall of the 

 follicle ; and as the ovum lies near this place 

 in the membrana granulosa, it is liable to be 

 evacuated first, along with a portion of that 

 membrane, which is soon torn away from the 

 rest by the pressure of the fluid behind it, 

 impelled by the contraction of the walls of 

 the follicle and surrounding ovarian substance. 

 During the descent of the ovum through the 

 Fallopian tube the cells of the proligerous disc 

 immediately surrounding the ovum alter their 

 form, and are subsequently detached from the 

 zona, so as to leave its external surface quite 

 free and smooth. I have not, any more 

 than BischofF, been able to observe the four 

 retaining straps or retinacula described by 

 Martin Barry in his first series of Researches, 

 as portions of the membrana granulosa na- 

 turally thicker than the rest, and which, 

 radiating nearly at right angles from the pro- 

 ligerous disc, serve, as it were, to guide the 

 ovum and its disc towards the aperture by 

 which it escapes on the bursting of the fol- 

 licle. The accompanying figure from Coste 

 (56*.) gives that author's view of a structure 

 somewhat similar to the retinacula of Barry. 

 The size of the mammiferous ovum itself is 

 much more uniform among the different families 

 of Mammalia than that of the Graafian follicle, 



