OF TIIE OVUM IN GENERAL, AS RELATED TO THE SEXUAL PROCESS OF GENERATION.
The term OVUM is in this article entirely restricted to the product of sexual genera tion. This body is formed in the ovary of the female parent (or in the female organ of a hermaphrodite parent) by a gradual process of growth or development. When it arrives at a state of maturity, it is spontaneously dis charged from the place of its formation, a process which in the higher animals has re ceived the name of Ovulation. If left to its own unassisted powers, no organic change of importance follows in the ovum, and it remains incapable of producing an embryo. But if, at or near the time when the ovum, in a state of maturity, leaves the ovary, it be sub jected to the influence of the male product or sperm by the contact of a very minute portion of that substance, it then undergoes the change of Fecundation, by ,ahich it has communicated to it the power of having de veloped within it a new being specifically resembling its parents.
Although there are many great apparent differences in the form and structure of the ova of animals*, yet a general comparison of their organisation shows that they consist in near/y all of parts that are essentially the same. These parts in the ovarian ovum are the following, beginning with that which appears most essential : 1st, The Germinal esicle, or Germ-cell ; a nucleated organ;e cell of microscopic size, generally situated near the surface of the ripe ovarian ovum : this is embedded superficially in, 2nd, The Vitellus, re/k, or Yolk, a mass of oleo-albu minous matter, partly fluid, and partly cellular and granular, generally of propor tionally much greater size than the germ-cell, and serving to furnish materials for the changes of that body, and for the develop ment of the new being. Both of these parts are enclosed by, 3rd, The Vitelline, or Y olk Membrane, a vesicular, nearly structureless, membrane, which contains the rest, and gives to the whole usually more or less of a sphe rical form. To the assemblage of these parts, constituting the ovarian ovum, and which may be looked upon as most immediately im portant in connection with the formative pro cess, there are generally added, after it has left the ovary, and in the progress of its descent through the female passages, some others, such as the albumen, outer membrane and shell of the bird's egg. In their simplest
form these additional parts constitute an ex ternal covering of the egg, to which the name of Chwion is often applied.
If the ovum be traced back to its earliest origin in the ovary, it is found to consist at first of the germinal vesicle, germ-cell or its nucleus (fig. 1, c.). To this cell the sub stance of the yolk is added in the progress of its formation, generally in a gradual manner, but in some animals more suddenly.
The Spermatic Substance, or SpCVM of the male, when examined in its state of maturity, as it is applied to the ovum, and effects in it the peculiar change of fecundation, is observed to consist essentially of an immense number of minute bodies, generally consisting of a thicker particle, with a fine filament attached, and almost always exhibiting, when recently mixed with water, vivid vibratory or undula tory movements, hut in a feW animals present ing other forms, and without motion. These spermatic filaments or particles are developed by a peculiar process in the interior of the cells (sperm-cells) secreted in the male organ or testis.
When the ovarian ovum has arrived at maturity, the germ cell disappears as such, and if fecundation shall have taken place, that vesicle is succeeded by another minute cell, with which the oricrin and development of the new animal are most intimately associated. This secondary organic cell of the fecundated ovum has therefore been called the Embryo cell. The first changes, preparatory to the commencement of the development of an embryo, consist in the formation out of the embryo-cell and yolk substance of an organised cellular mass, or of a membranous covering of the whole et. a part of the yolk: this is the germ-mass, Blastoderm, or germinal membrane, The process by which this primary organised part is produced varies somewhat in different animals; but it appears to consist in a mul tiplication of the embryo-cell by changes of the nature of cytogenesis, accompanied with more or less of a cleavage or sub-division of the substance of the yolk, and its com bination with the progeny of the embryo-cell. The general result is, that the first rudiments of the new being take their origin in organic cells, which are descended from the original embryo-cell.