RECAPITULATION AND CONCLUSION Having now stated in detail the principal facts that have come under our knowledge with regard to the form, structure, and mode of origin of the ova of different animals, it may be proper, in bringing this article to a close, to endeavour shortly to deduce from these facts the most general results to which they appear to lead. These results, together with some re flections on our subject, may be stated under the following heads, viz. 1. Definition of the ovum, as related to its own structure, and its history in connection with the reproduction of the species. 2. Recapitulation of the most general facts ascertained by the comparison of the ova of different animals. 3. Morphology of the ovum ; homology of its parts; and rela tion of the ovum to other organic structures. 4. Phenomena attendant on the maturation of the ovum. 5. Relation of the ovum to fecundation by the male sperm. 6. Immediate effects of fecundation on the ovum ; and re lation of the ovum after fecundation to the first commencement of the process of em bryonic development.
I. Definition of the ovum, as related to its own structure, and its history in connection with the reproduction of the species.
In the commencement of this article the ovum was shortly defined as " the product of parental sexual generation from which the young of animals are developed (produced)." This definition appears correct and sufficiently comprehensive ; but should it appear desirable to substitute for it a more precise description of the characteristics of the animal ovum, the following may be proposed as applicable to the ovum throughout the whole animal king dom, without involving any merely theore tical view of its structure and constitution, viz. " the ovum may be shortly described as a detached spheroidal mass of organised substance, of variable size, enclosed by a vesicular membrane, and containing in the earlier periods of its existence an internal cell or nucleus ; these parts, formed by the female individual or organ of animals, are capable, when fecundated by the male sperm of the same species, of giving rise, by the series of histogenetic and organogenetic changes known under the general term of develop ment, to an embryo, from which either directly or mediately the individuals of the animal species to which the parents belong are re produced."
We thus separate from the category of true ova all those bodies of an apparently reproductive kind which are not the direct product of an act of sexual generation. To such bodies, the nature of which is as yet doubtful, and probably somewhat various, the indefinite appellations of buds, bud-germs, gemnnw, spores, winter ova, ephippial ova, statoblasts, &c., have been given according to the circumstances in which they are se verally produced.
In all animals, then, with the exception of the Polygastric Infusoria and Rhizopoda, the occurrence of sexual generation and the for mation of true ova are proved to be the regular and constant means for the permanent reproduction or maintenance of the species. In the exceptional instances now mentioned, and even in some others possessed of the sexual distinction, the best known and most common multiplication of individuals takes place by a subdivision of the parent body, either by fissiparous cleaving or by gemma tion ; but in them also it can scarcely be doubted that there are other means by which the permanence of the species is maintained.
All the most accurate recent investigations lead to the conclusion that the production of the young of all organised beings, even the simplest of the Protozoa, does only occur by direct connection through some organised medium with other beings of a similar kind or species. We are forced, therefore, to con clude that in the propagation or production of these simple beings, in circumstances where their more ordinary fissiparous or gemmi parous mode of multiplication cannot be ad mitted to have taken place, there must have passed from the bodies of the progenitors minute particles of organised substance (ca pable, as we know, of being suspended in the atmosphere, and of resisting during a long period many of those influences which gene rally prove inimical to animal development), which particles, when brought into circum stances favourable to the progress of the vital processes, undergo the cycle of changes ne cessary for the reproduction of beings similar to those from which they sprang. If there is any constant law which seems more certainly than others to result from all recent researches into the history of organic nature, it is this necessary connection by descent of one being or set of beings from another.