From this blastodermic mass or membrane, the embryo, or fcetus, or new animal, and in the higher animals some accessory parts, which are temporarily united with the embryo previous to its birth, originate, and are gra dually formed, by a various process of pro gressive organic growth of an epigenetic character, which is termed Development, or Embryo-genesis.
In by far the greater number of animals an ovum gives rise to only one embryo or indi vidual, and this one becoines by itself, when its growth is complete, the perfect sexual animal, capable of contributing its share to the pro duction of fecundated ova. But in a certain number of animals, to Ahich allusion will be made more fully afterwards, the immediate product of development from the ovum is not at once, and by itself, converted into a com plete sexual individual ; but by an intermediate non-sexual process of production, one or more new individuals are formed out of the body of that first developed, and to the last so formed is committed the office of sexual reproduction, or true generation.
The essential conditions and phenomena, therefore, of the sexual process of generation, as related to the ovum, and as limited by the foregoing considerations, may be shortly stated to be the following:— 1st. The formation of the ovarian ovum of the female sex, containing the germ-cell.
2nd. The formation of the sperm-cells of the male sex, and the development of their peculiar spermatic elements.
3rd. The mutual action of these two pro ducts in the fecundation of the ovum.
4th. The disappearance of the germ-cell of the ovarian ovum, and the formation of the embryo-cell in the fecundated egg.
5th. The multiplication of the embryo-cell by cytogenesis, and the formation from that body, and from the yolk, or a part of it, of the blastodermic mass or membrane.
6th. The process of embryo-genesis, or development of the systems, organs, and textures of the new animal.
It is right to state that the original germ cell has not yet been ascertained to exist in the ovum of every animal,nor has its successor, the embryo-cell, been observed in all instances ; but they have been detected in so very large a proportion, that it appears extremely pro bable that in all sexual animals the generative process consists in the process above described, or in some modification of it. I refrain at
present from farther details as to these phe nomena, and have stated the results only in their most general form, because I shall have occasion to return upon some of them in a subsequent part of the article. Looking back on this general statement of the com mencement ancl progress of the genetic process in animals, it will be seen that the new being may be considered as taking its immediate origin from the progeny of cells descended from the embryo-cell. That cell appears with great probability to take its origin from the gertn-cell, or its nu cleus, or from some part of it, in combination with a determinate portion of the sperm product, or descendent of the sperm-cell ; and we are so far justified, therefore, in ascribing the genetic process by which the new being is formed to the mutual action of the products of two different kinds of cells, viz., the germ cell and the sperm-cell.* In conclusion, the ovum may be defined to be a distinct vesicular body originally formed from a ce I. presenting throughout its exist ence the organic cellular structure, consisting of oleo-albuminous materials, formed by the female of an animal species, and capable, when acted on by the spermatic product of the male, of undergoing the successive changes of embryo-genesis, by which, either directly or through intervening generations, the species of animals is reproduced and continued.
The structural distinctive characters of an ovum are, therefore, its enclosure within a distinct vesicular covering, and its original organic cellular constitution in the germ-cell : its most important physiological characteristic is its' susceptibility of the changes of embry onic development under the influence of the sperm-cell or its product.