Licence: Public Domain Mark
Credit: Ovum / by Allen Thomson. Source: Wellcome Collection.
Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.
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![rnals, whatever may be the ultimate structure of the yolk, the primitive yolk, or that which is first formed, is invariably of the finely gra- nular kind,—the cellular or large corpuscular yolk-substance is of later formation. These two parts remain distinct from each other, and the finely granular or formative yolk is that in which the germinal vesicle is invariably im- bedded. In those instances, such as the Bird, Reptile, &c., in which the large cellular yolk greatly preponderates over the formative yolk- substance, the latter assumes in the later stages of formation the shape of a flattish disc on one side of the greater mass of the yolk, with the germinal vesicle placed in its centre. The vitelline membrane presents some va- rieties in structure, being in some animals very delicate and homogeneous; in others, as Mammalia, remarkably thick, tough, and elastic, but without visible structure ; in a third set, exhibiting peculiar structure, such as the finely tubular perforations of the ex- ternal membrane of the fishes’ ovum, or the radiated markings in the ova of Holothiiria or Cestoidea ; but in these last three in- stances the vitelline membrane is probably associated with additional layers of substance derived from a different source from that which forms the homogeneous membrane. A remarkable peculiarity has recently been discovered in the enclosing membrane of the ovarian ovum of some animals, in the aper- ture or micropyle which has been observed in osseous fishes, insects, some Crustacea^, the Acephalous Mollusca, some Annelida, Holothuria, and some other Echinodermata. There seems reason to believe that a similar aperture exists in the ovum of Batrachia and Cephalopoda; and it is very probable that it may yet be discovered in other animals. At the same time it is right to state that in Mammalia and several other animals it has been most carefully sought for without suc- cess. This aperture appears to be designed to admit the spermatozoa into the cavity of the ovum, or into contact with the yolk-sub- stance and germ, in those instances especially in which the egg coverings are thick and tough, and fecundation is late of occurring. The relation of the ova to the ovaries or organs in which they are produced, exhibits considerable varieties in different animals. 1. The most common is that in which the germs of the ova arise within minute close follicles or vesicles, which are imbedded in the more or less solid or loose stroma of the ovary ; the follicle enlarging with the ovum as its other parts are added till the period of of maturity, when, periodically, the follicles open for the escape of the ova. 2. In a second form, as in Nematoid Worms and Insects, the germs of the ova are produced free in the upper part of an ovarian tube, * It has been inadvertently stated in a preceding part of this article _(p. IIG.) that the micropyle had not been observed in the ova of Crustacea, whereas Meissner has ascertained its presence in that of Gammarus. (See his Memoir in Zeitsch. fur Wissen. Zool. vol. V. p. 284.) and the yolk-substance, &c. are added gra- dually as the egg germs descend through suc- cessive portions of the tube : here no true de- hiscence is necessary to allow of the escape of the ova. 3. In a third form, as in Trema- tode and Cestoid Entozoa, distinct organs are provided for the formation of the ovigerms and the yolk-substance, and these last are brought together and combined into the sphe- rical form of an ovum in another part of the genital apparatus. 4. In the greater number of animals the germs for each ovum appear to arise singly, and the ova are thus isolated from the first; but it would appear that in some animals these germs arise in groups, perhaps by development from a common germ, so that they are from the earliest period connected together by pedicles. Yet, with all these differences, there is to be perceived, on the whole, a general similarity in the plan of formation of the parts of the ovum itself in different animals. This plan may be generally stated as follows. The germinal vesicle is universally the first part of the ovum which makes its appearance ; it does not appear to be nucleated or to pos- sess its macula from the first in all instances, and this macula cannot therefore be regarded as the centre of its formation. The germinal vesicle is generally at first only a minute point; it soon enlarges, however, and either possesses from the first, or at a very early period acquires, its macula or nucleus. In animals with the solid follicular ovary, each follicle is occupied by a single ovum, which begins within it as a minute germinal vesicle. The delicate wall of the follicle is also per- ceptible at the same time as the ovigerm ; in- deed, there is reason to believe that it even precedes the commencement of the formation of the ovum, though this is a point not yet fully determined. In those animals, on the other hand, in which the ovary is tubular, the ovigerms appear,in some instances at least, to arise in groups within cells; and it may be a question whether these cells bear to the ovi- germs arising within them the relation of the ovarian follicles of solid or closed ovaries. Whether this be so or not, that relation is in most instances speedily changed, as the ova soon become free, or, in others, are attached by a pedicle to a common stalk. The wall of the ovarian follicle consists at first of an extremely delicate vesicular mem- brane, which is the same as that to which 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 [k3]](https://iiif.wellcomecollection.org/image/b24918751_0139.jp2/full/800%2C/0/default.jpg)
