The wall of the ovarian follicle consists at first of an extremely delicate vesicular mem brane, which is the same as that to NI hich the name of ovicapsule or ovisac has been given, At a very early period,and while the ovum con sists of no more than the germinal vesicle, the homogeneous wall of the follicle is lined with a layer of flat cells somewhat analogous to some forms of epithelium : this is the com mencement of the structure which in Mam malia afterwards forms the tunica granulosa, and the fluid and cellular contents of the Graafian follicle, It appears to have various destinations in different animals.
The second stage in the formation of the ovum is the deposit of the vitelline substance around the germinal vesicle. In most ani mals the yolk-substance, when it first begins to be formed, is scarcely granular, and in some instances quite clear, consisting of a viscous blastema, and as it increases separating the ger minal vesicle within from the ovarian follicle, which expands proportionally. Very soon, however, and in many animals indeed from the first, fine opaque granules make their ap pearance, as if by precipitation or deposit, in the clearer basement substance, and thus the primitive yolk-substance of the ovum in all animals is formed. In most instances there is a time during which the ovum, consisting of germinal vesicle, with a small quantity of primitive yolk, exists, without any other co vering than that given to it by the ovarian follicle ; but as the deposit of the finely granular yolk increases, and at a very variable period in different animals, the vitelline mem brane is formed round its exterior. The ad dition of this covering may be regarded as the third stage in the fbrmation of the ovum. The manner of the origin of the vitelline membrane has not yet been accurately ob served ; and it is probable (as will be hereafter stated) that the coverings known under this name may have different modes of origin; but if we restrict our attention at present to such simple ova as those of Mammalia, I believe it may be stated as extremely probable that the so-called zona pellucida which constitutes the vitelline membrane of the Mammiferous ovum, takes its origin by the consodidation of the superficial part of the basement substance of the primitive yolk.
It appears probable that in the large-yolked ova, such as those of the bird, the vitelline membrane, which we find enclosing the whole mass of' the yolk, owes its origin to a dif ferent source ; and I am inclined to believe that in this and in many other animals the meinbrane which we term vitelline, as being the immediate investment of the yolk, is not of the same nature with the zona pellucida, or the simple homogeneous vesicle of the smaller ova, but rather a structure of later formation, which owes its origin to the fusion, or amalgamation, or to sotne other change in the outermost layer of cells which form the nutritive yolk of these animals.
In connection with this view, it is import ant to remark, that at that earlier stage of formation of the bird's egg when it consists entirely of formative or primitive yolk, there is an approach to the formation of a zona, in the existence of a very distinct, clear, and consistent marginal portion of the yolk bias tetna, from which the yolk granules seem to retire. When the large cellular or nutritive
yolk is formed, this temporary zona seems to disappear, and to be replaced externally by the permanent vitelline metnbrane already mentioned.
In those animals in which the ovigerms arise by development within cells so as to be connected in groups (Gordiacei), and in some others, the vitelline membrane, or a substitute for it, seems to be formed from the earliest period in a different manner from that now described.
The germinal vesicle is unimacular in ge, neral in the small-yolked ova, and multima cular in the large-yolked ova, and also in the intermediate kinds. In the latter it is rare to observe the earliest stage in which the ma cula is still single : the multiplication of the maculm takes place with remarkable rapidity, and apparently by a process of endogenous development, or possibly by division. The ultimate destination of these maculw is still a subject of doubt.
3. Morphology of the ovum ; homology of its parts, and relation of the ovum to other organic structures.
Should the views be correct which have now been stated with regard to the relations of the parts in the mature ovarian ovum, and the manner in which they are forined, it will be apparent that a strict homology or ana tomical correspondence can be pointed out in regard only to some of the parts which are recognised under similar designations, as re spectively belonging to the ova of different animals. All physiologists will probably be disposed to look upon the germinal vesicle or ovigerm as corresponding or homologous in the ova of all animals, and, notwithstanding the great differences known as to its more simple or multiple condition, the same view may also be taken of the structure known as nucleus or macula. The primitive or finely granular yolk-substance, more especially that which immediately surrounds the germinal vesicle, and is afterwards employed in the formation of the blastoderm or embryogerm, seems also to have a similar origin, structure, and relation in all animals. But beyond this it is more difficult to trace the homological correspondence ; for under the names of cellular yolk-substance and vitelline mem brane it appears that there have been brought together parts of which the origin, structure, and relations may be dissimilar in different animals. There seems at least to be sufficient reason, from what is already known of the varieties of the enclosing membrane, or so called vitelline membrane, to establish a dis tinction between several forms of that struc ture; as, for example, between the vitelline membrane, which exists from the earliest period as a pediculated sac in connection with the ovarium, as in Holothuria ; that which is derived from the extension of the wall of the original germ-cell in grouped ova, such as have been described by Meissner in Gordiacei ; that which is later formed round the ovum of Mammalia as a zona pellucida, by the consolidation of the outer layer of the primitive basement substance of the yolk ; and that which in the bird and other animals whose ova are similarly constituted, appears to derive its origin in part, at least, frotn coalesced cells corresponding to those of the tunica granulosa of the ovarian capsule on the exterior of the cellular yolk.