The membranes of the ovarian vesicle in mammalia and the capsules of the ovary in the fowl arc corresponding parts, and the prin cipal difference between the ovarian cavities containing ovula in oviparous animals, and those of viviparous animals, consists in this, that in the latter the ovulum (the vesicle of Baer) is placed in the granular proligerous disc, and has all the fluid of the vesicle interposed between it and the coats of this cavity.
At the time when Baer first discovered the ovulum of mammalia, there was still wanting, in order to complete the proof of its analogy with the ovulum of birds, the observation of the ger minal vesicle (vesicle of Purkinje) within it. This additional proof has been supplied within the last few years by the researches of T. NV. Jones, Coste, Purkinje, Valentin, and Wag ner, which we have ourselves confirmed.
The germinal vesicle of the vcry small ovu lum of quadrupeds is of course a most minute object, and in fact it can only be seen with a good microscope ; but in favourable circum stances it is nevertheless quite distinct, and the investigations above referred to, conjoined with analogical evidence, make it highly pro bable that the little vesicle found within the ovulum of viviparous animals occupies the place in which the foetus first makes its ap pearance, and that at the time of the passage of the ovulum from the ovary to the Fallopian tube this little vesicle is burst, and undergoes analogous changes to those which have been noticed in the fowl.* In birds the shell with its lining membrane forms the external covering of the egg ; and in all oviparous animals a similar external enve lope (besides the membrane enclosing the yolk) is to be found, though varying greatly in thick ness, consistence, and structure in different animals. The ovum of mammalia at the time when it arrives in the uterus has also a similar external envelope, which has received in man and most animals the general appellation of chorion. Baer is of opinion that the chorion exists ready formed in the ovulum of the ovary ; but his observations appear to us as yet insufficient to prove this point, and we feel inclined rather to adopt the view of Valentin, who holds that it is probable that the chorion is added to the ovulum after it has left the Graafian vesicle, that is, during its passage from the ovary to the uterus, somewhat in the same manlier as the albumen or shell is added to the egg of the common fowl in its passage through the oviduct. The analogy of all ovi parous animals is at least strongly in favour of such a view of the mode of the production of the chorion or external envelope; while on the other hand we ought not to lose sight of the fact that though the external envelope or cho rion occupies the same position as the external covering of the eggs of oviparous animals, its structure and functions are very different, for almost in every quadruped the chorion serves important purposes in establishing that more intimate union peculiar to viviparous animals, which is formed between the ovum and uterus in the placenta or analogous structure.
It is only in the dog and rabbit that the ova have hitherto been traced by actual observation in the whole course of their progress through the Fallopian tubes from the ovary to the uterus. These observations we owe chiefly to the care ful researches of Cruikshank, l'rcvost and Dumas, Baer, and Coste. In regard to other animals we have only a few detached observa tions in some of them, and in the human species the ova have never been observed in the Fallopian tubes, nor indeed for some time after they must have entered the uterus. We do not therefore know, with any degree of cer tainty, at what distance of time after sexual union the ovum passes into the uterus of the human female. Great difficulties attend the elucidation of this point. In the first place, we are opposed by the impossibility, in the greater number of cases in which we may hap pen to obtain a pregnant uterus for investiga tion, of knowing accurately the age of the product or the time at which impregnation has occurred; and in the second place, we are here deprived of the assistance derived in many other parts of our subject from analogical evidence by the wide discrepancies we find among animals in respect to the period of the arrival of the ova in the uterus ; for there does not appear to be any exact correspondence yet shewn between the time at which this happens and the length of duration of utero.gestation. It may be well, however, to endeavour to form an approxitnative opinion. In the rabbit, although ova are known frequently to be dis charged from the Graafian vesicles on the se cond day after sexual union, they are in general not detected in the uterus before the third or fourth day, and frequently not before the fifth or sixth, at which time they appear as vesicles a little more than a line in diameter, lying un attached in the upper part of the cornua of the uterus.* In the dog ova have been observed in the Fallopian tubes on the eighth day, but they have not been found in the uterus before the twelfth day. In the cat we have found ova of the size of peas beginning to be attached to the uterus at the twelfth day, and in both the cat and dog we think it probable from the size of the ova that they had already been in the uterus for at least one day, so that the tenth or eleventh day may be regarded as the time when ova generally appear in the uterus of these animals.