The works of Francis Maitland Balfour, Volume 1 (of 4)

by Francis M. Balfour

Fig. 1.—Unripe ovum of Toxopneustes lividus (copied from Hertwig).

Every ovum as it approaches maturity is found to be composed (Fig. 1) of (1) a protoplasmic body or vitellus usually containing yolk-spherules in suspension; (2) of a germinal vesicle or nucleus, containing (3) one or more germinal spots or nucleoli. It is with the germinal vesicle and its contents that we are especially concerned. This body at its full development has a more or less spherical shape, and is enveloped by a distinct membrane. Its contents are for the most part fluid, but may be more or less granular. Their most characteristic component is, however, a protoplasmic network which stretches from the germinal spot to the investing membrane, but is especially concentrated round the former (Fig. 1). The germinal spot forms a nearly homogeneous body, with frequently one or more vacuoles. It occupies an often excentric position within the germinal vesicle, and is usually rendered very conspicuous by its high refrangibility. In many instances it has been shewn to be capable of amœboid movements (Auerbach, and Os. Hertwig), and is moreover more solid and more strongly tinged by colouring reagents than the remaining constituents of the germinal vesicle. These peculiarities have caused the matter of which it is composed to be distinguished by Auerbach and Hertwig as nuclear substance.

In many instances there is only one germinal spot, or one main spot, and two or three accessory smaller spots. In other cases, e.g. Osseous Fish, there are a large number of nearly equal germinal spots. The eggs which have been most investigated with reference to the changes of germinal vesicle are those with a single germinal spot, and it is with these that I shall have more especially to deal in the sequel.

The germinal vesicle occupies in the first instance a central position in the ovum, but at maturity is almost always found in close proximity to the surface. Its change of position in a large number of instances is accomplished during the growth of the ovum in the ovary, but in other cases does not take place till the ovum has been laid.

The questions which many investigators have recently set themselves to answer are the two following:—(1) What becomes of the germinal vesicle when the ovum is ready to be impregnated? (2) Is any part of it present in the ovum at the commencement of segmentation? According to their answers to these questions the older embryologists roughly fall into two groups: (1) By one set the germinal vesicle is stated to completely disappear and not to be genetically connected with the subsequent nuclei of the embryo. (2) According to the other set it remains in the ovum and by successive divisions forms the parent nucleus of all the nuclei in the body of the embryo. Though the second of these views has been supported by several very distinguished names the first view was without doubt the one most generally entertained, and Haeckel (though from his own observations he was originally a supporter of the second view) has even enunciated the theory that there exists an anuclear stage, after the disappearance of the germinal vesicle, which he regards as an embryonic repetition of the monad condition of the Protozoa.

While the supporters of the first view agree as to the disappearance of the germinal vesicle they differ considerably as to the manner of this occurrence. Some are of opinion that the vesicle simply vanishes, its contents being absorbed in the ovum; others that it is ejected from the ovum and appears as the polar cell or body, or Richtungskörper of the Germans—a small body which is often found situated in the space between the ovum and its membrane, and derives its name from retaining a constant position in relation to the ovum, and thus serving as a guide in determining the similar parts of the embryo through the different stages. The researches of Oellacher (15)[363] in this direction deserve special mention, as having in a sense formed the foundation of the modern views upon this subject. By a series of careful observations upon the egg of the trout and subsequently of the bird, he demonstrated that the germinal vesicle of the ovum, while still in the ovary, underwent partial degeneration and eventually became ejected. His observations were made to a great extent by means of sections, and the general accuracy of his results is fairly certain, but the nature of the eggs he worked on, as well as other causes, prevented his obtaining so deep an insight into the phenomena accompanying the ejection of the germinal vesicle as has since been possible. Lovén, Flemming (6), and others have been led by their investigations to adopt views similar in the main to Oellacher's. As a rule, however, it is held by believers in the disappearance of the germinal vesicle that it becomes simply absorbed, and many very accurate accounts, so far as they go, have been given of the gradual atrophy of the germinal vesicle. The description of Kleinenberg (14) for Hydra, and Götte for Bombinator, may perhaps be selected as especially complete in this respect; in both instances the germinal vesicle commences to atrophy at a relatively early period.

Coming to the more modern period the researches of five workers, viz. Bütschli, E. van Beneden, Fol, Hertwig, and Strasburger have especially thrown light upon this difficult subject. It is now hardly open to doubt that while part of the germinal vesicle is concerned in the formation of the polar cell or cells, when such are present, and is therefore ejected from the ovum, part also remains in the ovum and forms a nuclear body which will be spoken of as the female pronucleus, the fate of which is recorded in the second part of this paper. The researches of Bütschli and van Beneden have been especially instrumental in demonstrating the relation between the polar bodies and the germinal vesicle, and those of Hertwig and Fol, in shewing that part of the germinal vesicle remained in the ovum. It must not, however, be supposed that the results of these authors are fully substantiated, or that all the questions connected with these phenomena are settled. The statements we have are in many points opposed and contradictory, and there is much that is still very obscure.

In the sequel an account is first given of the researches of the above-named authors, followed by a statement of those results which appear to me the most probable.

The researches of van Beneden (3 and 4) were made on the ovum of the rabbit and of Asterias, and from his observations on both these widely separated forms he has been led to conclude that the germinal vesicle is either ejected or absorbed, but that it has in no case a genetic connection with the first segmentation sphere. He gives the following description of the changes in the rabbit's ovum. The germinal vesicle is enclosed by a membrane, and contains one main germinal spot, and a few accessory ones, together with a granular material which he calls nucleoplasma, which affects, as is usual in nuclei, a reticular arrangement. The remaining space in the vesicle is filled by a clear fluid. As the ovum approaches maturity the germinal vesicle assumes an excentric position, and fuses with the peripheral layer of the egg to constitute the cicatricular lens. The germinal spot next travels to the surface of the cicatricular lens and forms the nuclear disc: at the same time the membrane of the germinal vesicle vanishes though it probably unites with the nuclear disc. The nucleoplasma then collects into a definite mass and forms the nucleoplasmic body. Finally the nuclear disc assumes an ellipsoidal form and becomes the nuclear body. Nothing is now left of the original germinal vesicle but the nuclear body and the nucleoplasmic body both still situated within the ovum. In the next stage no trace of the germinal vesicle can be detected in the ovum, but outside it, close to the point where the modified remnants of the vesicle were previously situated, there is present a polar body which is composed of two parts, one of which stains deeply and resembles the nuclear body, and the other does not stain but is similar to the nucleoplasmic body. Van Beneden concludes that the polar bodies are the two ejected products of the germinal vesicle. In the case of Asterias, van Beneden has not observed the mode of formation of the polar bodies, and mainly gives an account of the atrophy of the germinal vesicle, but adds very little to what was already known to us from Kleinenberg's (14) earlier observations. He describes with precision the breaking up of the germinal spot into fragments and its eventual disappearance.