Our design at present is to follow the ovum only as far as into the uterus, or until the com mencement of the formation of the foetus in it. We believe we shall place this part of our sub ject in the clearest point of view, by prefixing to our remarks regarding the ovum of man and quadrupeds a short sketch of what happens to the egg of the common fowl after its dis charge from the ovary.
The substance of the yolk enclosed in its membrane, together with the germinal portion in which after incubation the rudiments of the new animal begin to be formed, constitutes the essential parts of the bird's egg as it exists in the ovary. The ovarian egg, when it has left the place of its formation and passed into the oviduct, receives the addition of various other parts, viz. the albumen, chalazw, shell and its lining membrane, as itgradually descends through different portions of the oviduct, each of which is destined to secrete one of these newly added parts. These parts may, however, be considered as accessory to the more essential constituents of the egg, which we are inclined to regard as the germinal spot. or cicatricula, the granular and oleaginous fluid of the yolk, and the dense transparent membrane with which they are enveloped. To the unim pregnated egg of the ovary we shall give the name of ovulum, and henceforward in this paper apply the name of ovum to the perfected egg, that is, the ovulum to which the acces sory coverings have been added, and which has received the influence of the male. The ovarium of the common fowl in the breeding season, or when it is laying eggs, has the form of a bunch of clustering ovula, which are contained in capsules, the more advanced of which hang down from the rest of the ovary by the elongated pedicles of the containing capsules ; while the smaller ovula of various sizes, composing the body of the ovary, cluster more closely together. The fully developed ovula only have the deep yellow colour pecu liar to the yolk ; as the smaller ones are less advanced their colour is paler, and the smallest are nearly colourless and transparent from the absence in them of the oleaginous and granular matter peculiar to the riper yolks.
The little white spot or granular layer which constitutes the cieatricula or germinal disc is easily seen in the larger ovula, occupying almost always the same position on the surface of the yolk, somewhere near the pedicle of the ovarian capsule. When the cicatricula is examined carefully in the ovulum, a small dark round spot is perceived in its centre, the relations of which to the first production of the foetus are very important. This little dark spot was discovered by Purkinje to contain im planted in the centre of the cicatricula a minute transparent vesicle filled with fluid. He farther sheaved that during the passage of the ovulum from the capsule of the ovary into the infun dibulum of the oviduct, this little vesicle dis appears, being probably burst, and leaves in its place a thin and tender transparent mem brane. The vesicle of Purkinje, as it is called
from its discoverer, occupies then the centre of the germinal spot, and it is in the transparent membrane left in its place when the vesicle is dispelled that the first rudiments of the feetus afterwards make their appearance. Hence the vesicle has also received the name of germinal vesicle, a most appropriate term, since it may be regarded as the more immediate seat of the germ or germinating faculty of the egg.
The Purkinjean or germinal vesicle exists in the smallest as well as in the more advanced ovula of the fowl's ovary, and it is proportion ally much larger in small than in large ovula. In the very small ovula it is not, as in the riper ones, situated on the surface of the yolk, but towards the centre of that body; and as the ovu lum advances to perfection, the germinal vesicle gradually approaches more near the surface, and becomes more prominent on the surface of the cicatricula. In ovula less than two lines in diameter the vesicle is usually unconnected with the germinal layer or cicatricula, but in those of four lines in diameter it is already placed in the middle of the germinal spot.
In all oviparous animals a vesicle, simi lar to that now described in the common fowl, occupies the central part of the germinal layer so long as the ovulum remains in the ovary, and undergoes the same rupture and other changes at the time of the discharge of the ovulum from the ovary, In turning now to mammiferous or vivipa rous animals, it may be remarked in the first place, that although the extremely minute size of the body discovered by Baer to be constantly present in the ovarian vesicle prevents us from observing it with ease, and establishing with certainty its analogy to the yolk and its accom panying parts in the eie of the fowl before deve lopment begins; yet after the commencement of festal formation, the early changes which this body undergoes prove its correspondence with the ovum of birds in a most satisfactory manner. We have already, however, stated the reasons for regarding the vesicle of Baer as the ovulum of inaminalia, and need not now recapitulate them. We shall only remark that although the vesicle of Baer and ovulum of birds differ widely in size, that vesicle appears to contain the same essential parts of the egg belonging to birds and other oviparous animals, viz. a fluid granular mass or yulk enclosed by an in vesting membrane, and furnished also with a more compact granular layer situated on the surface of the yolk, but also enveloped by its membrane, in which the rudiments of the fcetus first appear, and which is, therefore, the germinal layer of the mammiferous ovum.