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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![[136] rent isolation of the origin of cells in blastema or intercellular substance, it might still be held that the unseen germs of new cells con- tained in that blastema may have derived their origin from other cells or organised parts proceeding from cells. And thus, in regard to the first origin of the ova of animals, it is fair to conjecture that the germs from which they spring have taken their descent from parent cells or structures derived from cells through the organs appropriate to their form- ation, But here observation fails to assist us further, and we are lost in the region of speculation. If, however, with the reservations now stated, it should be thought desirable to compare the ovum to the organic cellular structures, the germinal vesicle may be re- garded as the simple cell of the ovum, the whole ovum as a complex cell; the first of these being formed probably by expansion from a minute point or molecule, the second by superposition or external deposit round the internal cell ; but both at the same time presenting features which are peculiar to themselves, and different from those which characterise other cells of the animal eco- nomy. The different and separate formation of the germinal vesicle and yolk, which is perceptible to some extent in the ova of most animals, is placed in its most striking point of view by those instances in which, as in Tre- ’ matode and Cestoid Entozoa, there are dis- tinct germigeiious and vitelligenous organs, and those in which, as in ISematoidea and Insecta, the ovary is tubular, and the forma- tion of the several parts of the ovum goes on progressively in different parts of the tube. 4. Phenomena attendant on the maturation of the ovum, and its discharge from the ovary. The ovum naturally undergoes in the ovary a progressive development till it arrives at the state of maturity, when it is usually separated from the ovary by a process of dehiscence, is conducted through the female passages either to be excluded or laid, as in oviparous ani- mals, or to be retained in a uterus or other part of the female organs in viviparous ani- mals during uterogestation. The maturation of the ova and their separation from the ovary is in many animals periodical and inde- pendent of fecundation. This natural peri- odical separation of the ova has been termed Ovulation by some authors.* The change which the germinal vesicle undergoes at the period of the maturation of * The observations of Bischoff had long ago shown that in the periodical dehiscence of ova which accompanies the heat of female quadrupeds, the ova may be detected, though unfecunclated, in the course of their descent through the Fallopian tubes and uterus (Periodische Losreifung, &c., 1842), and some observations appear also to have shown that the same is the case in the human female at the periodical return of menstruation. (See a paper by H. Letheby, M. B. in the Philos. Trans, for 1851, p. 57., where two cases are described in which ovules or their remains were detected in the Fallopian tubes of unimpregnated women who had died at or about the menstrual period.) the ovum has naturally attracted much at- tention, from the hope that through the ob- servation of this phenomenon some explanation might be obtained of the first origin of the germ round which, after fecundation has taken place, the segmenting and organising stratum is collected, from which the blastoderm is produced ; but it must be allowed that as yet little success has attended our efforts to de- tect the connection, if it exists, between these two processes. In almost all animals the germinal vesicle is lost to view at the time of the maturation of the ovum, and generally before or about the time when the ovum leaves the ovary. In large-yolked ova the maculae of the germinal vesicle become very numerous by their multiplication and sub-divi- sion at an early period ; while in the small- yolked ova, as has been observed in a few animals at least, the increase in the number of the maculae does not take place till imme- diately before the diffluence or disappearance of the vesicle. The more minute phenomena of this diffluence are as yet very imperfectly known. In some animals, as Mammalia and Birds, it has been observed that shortly before the diffluence of the vesicle, its delicate wall undergoes a softening or approaching solution, so as to make it impossible to separate the vesicle entire. After this, when the diffluence is complete, the contents dis- appear from the situation they have previously occupied, but what becomes of them has not yet been determined. In some instances, as Birds and Batrachia, it has been observed that, after the diffluence of the germinal ve- sicle, the germinal part of the yolk, which previously consisted exclusively of small opaque granules, is now mingled throughout with clear hyaline spherules, somewhat similar to the dispersed maculse of the germinal vesicle; but ft is only matter of conjecture that these clear spherules have been derived from the germinal vesicle or its maculae. In a few instances, as in Ascaris, it has been thought that the entire nucleus or macula of the germinal vesicle has remained undivided, and it has been supposed that it has of itself constituted the germ of the embryo-cell, which afterwards occupies the centre of the first segmenting mass of the yolk, and whose progeny by division exists as nuclei in the interior of the successively in- creasing segments of the cleaving germinal portion of the yolk. A recent observation by J. Muller seems to lead the way to a different view of this phenomenon. He has observed * in one of the Mollusca, the Ento- choncha mirabilis, that the germinal vesicle does not disappear or undergo a change at the time of the maturation of the ovum, but remains discernible as the foundation of the clear embryonic-cell which occupies the centre of the yolk mass when segmentation is about to take place. Reinak f has been led, by his observations on the Batrachian ovum, to * Archiv. der Phj^siol. 1852. Leydig in the same. + Untersiich. iiber die Eutwickel. der Wir- berthiere.](https://iiif.wellcomecollection.org/image/b24918751_0142.jp2/full/800%2C/0/default.jpg)
