[90] 



OVUM. 



the membrane or wall of the follicle, and all 

 that yet exists of the ovule. Next, the yolk 

 substance is formed round the germinal vesicle ; 

 first of all, as has been shown by Leuckartf, 

 by the deposit of a clear viscid fluid, and 

 next by the formation of dark or opaque small 

 granules in this fluid adjacent to the germinal 

 vesicle. Somewhat later the zona pellucida, 



Fig. 63 *. 



Ovarian follicle and ovum of the Rabbit at an early 

 stage. 



The follicle here represented was about yL- in 

 diameter : in the figure it is shown as it appears 

 tinder slight pressure. All the parts of the ovum 

 are distinct, and its large size in proportion to the 

 follicle and tunica granulosa is apparent. In the 

 lower of the two figures the follicle is represented 

 as having been burst by pressure and the ovum 

 with the tunica granulosa in the act of escaping 

 from within : the yielding character and elasticity 

 of the zona is shown by the change of form during 

 the escape, and the ovum afterwards regaining its 

 spherical shape, o, the wall of the follicle ; t g, the 

 tunica granulosa; z, the zona partially freed from 

 the cell covering. The macula with the germinal 

 vesicle is remarkably distinct, and is surrounded 

 by a quantity of fine molecular substance. 



which cannot be said to have existed from the 

 first, comes to be apparent outside the opaque 

 granular substance of the yolk, and close to 

 the epithelial cells which line the follicle: it 

 seems as if it owed its origin to the condensa- 

 tion of the outermost layer of the clear base- 

 ment matter from which the yolk-substance 

 is formed. 



The membrana granulosa consists for a time 

 of a single layer of nucleated epithelial cells 

 situated between the ovule and the Graafian 

 follicle. The latter not being yet expanded by 

 fluid, is at this period completely filled by the 

 ovum. Such is the state of the parts, now all 

 present, in follicles of from T Q to jnyj/' in dia- 

 meter. Subsequently the follicle increases in 

 size much more rapidly than the ovum ; the 

 membrana granulosa follows closely the wall 

 of the follicle in its rapid expansion by the 

 increasing accumulation of fluid within ; and 

 the ovum is now found to be imbedded in 

 a particular portion of the layer of granular 

 cells constituting the cumulus. 



According to the best observations, then, 

 as to the formation of the mammiferous ovum, 

 it appears that the ovarian follicle, which we 

 may look upon as the primary gland cell, is 

 first produced ; that within it at a very 

 early period the germinal vesicle with its nu- 

 cleus next arises, and that very soon after the 

 origin of this primary part the yolk-substance 

 commences by a deposit of fluid and granules 

 round the germinal vesicle ; that the Graa- 

 fian follicle is lined by a layer of nucleated 

 cells resembling epithelium, which constitute 

 the commencement of the tunica granulosa ; 

 that the zona pellucida, which forms the outer- 

 most covering of the ovum when it leaves the 

 ovary, is formed at an early period, but some- 

 what later than the commencement of the 

 other parts of the ovum ; and that it probably 

 owes its origin to a membranous condensation 

 of the outermost part of the clear primitive 

 yolk-substance ; and that, finally, the tunica 

 granulosa increases in quantity and extent, is 

 expanded along with the follicle by the fluid 

 within it, and being deposited at its thickened 

 cumulus round the ovum encloses it in a part 

 of its substance. 



The structure of the ovum is, on the whole, 

 very similar throughout all the families of the 

 class Mammalia in which it has been examined, 

 excepting one, viz., the Monotremata. In 

 Marsupialia, in which, from the remarkable 

 deviation from the more common mode of 

 gestation, it has been supposed that the ovum 

 might present some peculiarities, it does not 

 appear, from the observations of Professor 

 Owen, that any remarkable difference is to be 

 detected. In the ovisac there was observed 

 a somewhat larger quantity of granular sub- 

 stance than usual ; but the diameter of one of 

 the largest ova in the Macropus Parryi or 

 Kangaroo was not greater than T^", and the 

 germinal vesicle was only T e*g-t/', which is 

 proportionally small ; so that it cannot be held 

 that in this animal there 



was apparent any 



+ Article Zeugung in Wagner's Hand wort erbuch approach to the oviparous type, 

 der Physiologic, 1853. In the Monotremata, however, the ovum is 



