The germinal vesicle it might be held, both in its structure and its mode of origin, merits, more justly than the whole ovum, the com parison with an organised cell. But even in its history there are points of difference, and we are still too little acquainted with the mode and consequences of its disappearance at the time of the maturation of the ovum, to warrant our making more than a vague and general comparison of the germinal vesicle to an organised cell. In admitting that the ovum, or its germinal vesicle, present some of the features of the organic cellular structure, we ought always to bear in mind that they are the source of all the other cells from which the animal body is developed.
The manner of the very first origin of the germ of the ovum is still involved in obscurity, for we only know of the existence of an ovi germ when the germinal vesicle has attained an appreciable size. Whence the first germs of the germinal vesicles proceed can as yet be matter only of conjecture. Without enter ing here upon the debated ground of the origin of organised cells in general*, I would venture to express the opinion, that as there is no ovigerm without a parent, so there is no new organisation without previously existing, and at some time or other connected, orga nisation. Hence, notwithstanding the appa 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 germigenous and vitelligenous organs, and those in which, as in 1N ematoidea 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 inaturation 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 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 maculm 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 maculm does not take place till imme diately before the diffluence or disappearance of the vesicle. The more minute phenomena of this diffluenee 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 maculm of the germinal vesicle ; but it is only matter of conjecture that these clear spherules have been derived from the germinal vesicle or its maculm.