494? Royal Society. 



produce changes in the unimpregnated ovum, which in some respects 

 resemble those referable to impregnation. During the rut, the num- 

 ber of Graafian vesicles appearing to become prepared for dischar- 

 ging their ova, exceeds the number of those vi^hich actually discharge 

 them. Ova of the rabbit which are destined to be developed, are in 

 most instances discharged from the ovary in the course of nine or ten 

 howxi post coif urn; and they are all discharged about the same time. 



There is no condition of the ovum uniform in all respects which 

 can be pointed out as the j^articular state in which it is discharged 

 from the ovary ; but its condition is in several respects very different 

 from that of the mature ovum ante coitum. Among the changes 

 occurring in the ovum before it leaves the ovary, are the follow- 

 ing : viz. the germinal spot, previously on the inner surface, passes 

 to the centre of the germinal vesicle ; the germinal vesicle, previously 

 at the surface, returns to the centre of the yelk ; and the membrane 

 investing the yelk, previously extremely thin, suddenly thickens. 

 Such changes render it highly probable that the ovary is the usual 

 seat of impregnation. The author considers this view as being not 

 incompatible with the doctrine that contact between the seminal 

 fluid and the ovum is essential to impregnation, since he has found, 

 in the course of his researches, that spermatozoa penetrate as far as 

 to the surface of the ovary. The retinacula and tunica granulosa are 

 the parts acted upon by the vis a terijo, which expels the ovum from 

 the ovary. These parts are discharged with the ovum, render its 

 escape gradual, probably facilitate its passage into the Fallopian tube, 

 and appear to be the bearers of fluid for the immediate imbibition of 

 the ovum. After the discharge of the ovum from the ovary, the 

 ovisac is obtainable free from the vascular covering, which, together 

 with the ovisac, had constituted the Graafian vesicle. It is the 

 vascular covering of the ovisac which becomes the corpus luteum. 

 Many ova, both mature and immature, disappear at this time by ab- 

 sorption. In some animals minute ovisacs are found in the infundi- 

 bulum, the discharge of which from the ovary appears referable to 

 the rupture of large Graafian vesicles, in the parietes or neighbour- 

 hood of which those ovisacs had been situated. 



The diameter of the rabbit's ovum, when it leaves the ovary, does 

 not generally exceed the 135th part of an inch, and in some in- 

 stances it is still smaller. The ovum enters the uterus in a state 

 very different from that in which it leaves the ovary ; hence the 

 opinion, that "in their passage through the tube the ova of Mam- 

 malia undergo scarcely any metamorphosis at all," is erroneous. 

 Among the changes taking place in the ovum during its passage 

 through the Fallopian tube are the following; viz. 1. An outer 

 membrane, the chorion, becomes visible. 2. The membrane origin- 

 ally investing the yelk, which had suddenly thickened, disappears 

 hy liquefaction ; so that the yelk is now immediately surrounded by 

 the thick transparent membrane of the ovarian ovum. 3. In the 

 centre of the yelk, that is, in the situation to whicli the germinal 

 vesicle returned before the ovum left the ovary, there arise several 

 very large and exceedingly transparent vesicles : these disappear, 



