Lastly, the ovum may be considered as having two phases or stages of existence ; the one in connection only with the female parent or female organ, in which the greater part of the organised material first to be employed in development is provided, and in which the ovum arrives at a certain stage of maturity ; and the other in its relation to fe cundation, or to the influence of the product of the male by which its developing powers are awakened or called forth. The mature ovarian ovum may therefore, in one sense, be looked upon as complete, if we regard only its own structure ; but here its progress would be arrested without the occurrence of fe cundation, and if we view it, therefore, with reference to its more important destination as the means of continuing the animal species, the ovum can onlybe regarded as perfect when that hitherto inscrutable change has been effected on its substance by admixture with the minute elements of the sperm in fecundation. The constancy of this law in the whole animal king dom, with the exception of those of the Pro tozoa already referred to, makes it proper that our definition should make reference to fecund ation as the means of perfecting the ovum. To the nature of this process itself a further al lusion will hereafter be made.
2. Recapitulation of the most general facts ascertained by the comparison of the ova of different animals.
The ova of animals in their complete state may be considered as consisting of two sets of parts which are of very different relative importance in connection with the develop ment of the embryo : the first of these sets of parts belong to the ovarian ovum, and are formed previous to their quitting that organ ; the others are subsequently formed, and may be looked upon as accessory. These last often present great varieties, so as to cause the ex ternal form and appearance of the ova of ani mals to differ widely, while the ovarian part much more nearly corresponds. To this ovarium ovum we shall principally confine our present remarks.
An extended comparison of the ovarian ova of animals belonging to almost every family of the animal kingdom has shown that, notwithstanding great differences in size, and some variation in form and structure, they all agree in consisting of three essential and nearly similar parts before the period of their detach ment from the ovary : these are, 1st, The in ternal nucleated cell or germinal vesicle with its macula or maculm ; 2nd, The vitellus, or yolk-substance ; and 3rd, The enclosing vesi cular envelope, or vitelline membrane. In all animals there is, also, a general similarity in the manner in which these parts are formed and combined so as to constitute the ovarian ovum ; the germinal vesicle is the first produced, and may be regarded as the ovigerm ; the yolk substance next gradually envelopes it or is deposited round the germinal vesicle, and in general the vitelline membrane which encloses the whole is the latest formed.
The most marked differences among the ova of animals are connected with the struc ture of the yolk and the relation which it bears to the formation of the germinal part out of which the embryo is afterwards developed' Founding upon this difference, three groups' two principal and one subordinate, may be distinguished among the ova of animals :— 1st, The group of small-yolked ova, to which belong those of Mammalia and a considerable number of invertebrate animals, such as most Mollusca, the lower Crustacea, most Anne lida, the Entozoa, Rotifera, Echinodermata, Acalepha, and Polypina. in this group, the ovum is generally of small or of moderate size, as a whole ; the vitelline substance con sists entirely or chiefly of fluid with fine gra nular particles, and the entire yolk undergoes segmentation : the entire yolk mass, therefore, is directly formative, or is employed from the first in the formation of the blastoderm or organised substratum in which the embryo is developed : the germinal vesicle is in this group usually of small size, and has only a single macula, or one composed of very few particles.
The second principal group comprehends the large-yolked ova, such as those of Birds, Scaly Reptiles, Cartilaginous Fishes, and the Cephalopoda, to which, perhaps, may be added Insects, Arachnida, and most Crustacea. In this group, the largely developed yolk contains, suspended in its basement, homogeneous sub stance, two kinds of organised corpuscles, viz., 1st, A certain portion of the small granular part, similar to that of the small yolked ova, which occupies a limited but determinate place in the ovum, and in its centre the germinal ve sicle is situated ; and 2nd, A larger portion of spherules, cell-like or other corpuscles ofgreater magnitude. It is the first or finely granular portion only which is immediately germinal, or which is subject to segmentation and forms the basis of the blastoderm ; the remainder, or large cellular portion, is only secondarily employed in supplying nourishment to the embryo or its accompanying organised parts in the progress of their development. In the ova of this group, therefore, we distinguish the formative or directly germinal portion of the yolk-substance from the indirectly nutritive portion. In these ova, the germinal vesicle is also proportionally large, and the contents of the vesicle, though consisting in the earliest stages of their formation of a single macula, or of a very small number, very soon become converted into very numerous maculm, or into a fine granular pulp.