IMPREGNATION AND CONCEPTION—GENERAL ACCOUNT OF MAMMALIAN DEVELOPMENT.
the ovum itself, about the time of attaining maturity, certain changes of importance occur. Owing to the extreme difficulty of obtaining material at the proper time and in suitable condition for micro scopic investigation, these changes have not yet been seen to occur in the human ovum; but inasmuch as they are now known to take place in nearly all the groups of invertebrate animals, and have also been de scribed, though less completely, in fish, amphibians, and mammals, there is hardly any room for doubting that they occur in man also.
These changes, which appear to take place in mammals about the time of rupture of the Graafian follicle and escape of the ovum, are best known to us through the researches of Ed. van Beneden on the rabbit's ovum. The ovum first contracts slightly, so that it no longer completely fills the zona pellucida (vide Fig. 91). The germinal vesicle, which had previously oc cupied a central or only slightly eccentric position, now travels to the surface of the ovum; the delicate membrane enclosing the germinal ves icle disappears, and the contents of the membrane—i.e., the nuclear reticulum and the germinal spot—become modified so as to form what is called the nuclear body, which is situated within the ovum, but close to its surface. A little later, part of this nuclear body is ejected from the egg, and forms two small "polar bodies" (Fig. 91) lying in the space between the zona pellucida and the ovum, formed by the shrinking of the latter, as already noticed.
Van Beneden held that the whole of the nuclear body was bodily ex truded from the egg; but from the analogy of other animals it is in the highest degree probable that only a portion is extruded, and that the re mainder stays within the egg, and, travelling towards its centre, forms what is called the female pronucleus.
The upshot of this process is, practically, that the germinal vesicle moves from the centre to the surface of the egg, and that, after undergo ing certain changes, part of it is bodily extruded from the egg while the remainder stays within it. The change is an exceedingly important one,
because it appears that, after the polar bodies are extruded, the ovum loses all power of further development. We have seen above that the ovum is a single cell, which originally formed part of the germinal epi thelium, and that the germinal vesicle and germinal spot bear the same re lation to the whole ovum that the nucleus and nucleolus do to an ordinary epithelial cell. Now, in the ordinary process of multiplication of cells by division, the nuclei are known to play a very important part; when a cell is about to divide into two the nucleus divides first, so that we have a stage in which there is a single cell with two nuclei, and then, later on, the whole cell divides into two halves, each containing half of the original nucleus. From this it appears that the nucleus is the part of the cell specially concerned with the process of reproduction or multiplication, and the part in which that process is initiated.
The formation of the embryo from the ovum is, as we shall see imme diately, essentially a process of cell multiplication by division, in which the nucleus of the ovum plays exactly the same part as the nucleus of an ordinary epithelial cell. An ovum with an entire germinal vesicle or nucleus must be supposed to have in itself the power of multiplication by fission, and so of producing an embryo, and, as we see in the case of many insects, such an ovum has actually this power; but after the extrusion of the polar bodies, the ovum is left with an imperfect nucleus, and is no longer capable of multiplication, unless the portion of the nucleus that has been extruded is replaced.