In a few instances, as in Ascaris, it has been thought that the entire nucleus or macula of the germinal vesicle has remained undivided, and it has been supposed that it has of itself constituted the germ of the embryo-cell, which afterwards occupies the centre of the first segmenting mass of the yolk, and whose progeny by division exists as nuclei in the interior of the successively, in creasing segments of the cleavirw germinal portion of the yolk. A recent observation by J. Miiller seems to lead the way to a different view of this phenomenon. He has observed * in one of the Mollusca, the Ento choncha mirabilis, that the germinal vesicle does not disappear or undergo a change at the time of the maturation of the ovum, but remains discernible as the foundation of the clear emb)yonic-cell which occupies the centre of the yolk mass when segmentation is about to take place. Remak t has been led, by his observations on the Batrachian ovum, to doubt the correctness of the view hitherto generally adopted as to the entire disappear ance of the germinal vesicle in that instance, and holds it as probable that a part of it at least remains in connection with the forma tion of the embryonic cell. These statements are sufficient to show that the phenomena of the dehiscence of the germinal vesicle and its relation to the subsequent changes in the ovum induced by fecundation are as yet very imperfectly understood, and that the discovery has still to be made of the link in the chain of connection between the last stage of existence of the ovigerm, and the first origin of the nucleus round which the subsequent organising process of segmentation begins. But that some such connection exists, all who have made a study of this part of the his tory of the ovum are inclined to believe.
5. Relation of the ovum to fecundation by the male sperm.
The act of fecundation is necessary for the perfection of all true ova. In the production of gemmm or buds, in the multiplication of nonsexual individuals, and in the various examples of Metagenesis previously referred to, the germs from which the new products arise may be nucleated cells or groups of these, and may without doubt be the descend ants of the original cell-germs of ova ; but for their development into the new beings produced from them, no combination, sir far as is yet known, with the product of cells of a different kind, as in fecundation, is necessary. It is otherwise with all true ova. Their germs may be the descendants through the ovary of an original cell-germ, from which the animal bearing the ovary was produced ; but for the generation of an ovum the ovigerm must be subjected to the influence of the sperm, and for its development there is re quired a new process of organisation, inaugu rated by segmentation, which is the invariable consequence of fecundation, and is the first obvious change in a fecundated ovum leading to embryonic formation.
The developed form of the spermatic sub stance* is in by far the greater number of ani mals that of mi nute ovoid or rounded particles of various form, with each of which is connected a long and extremely delicate filament, which moves with vivacity in a vibrating or oscil latory manner when immersed in water and various bland animal solutions. There are other less common forms of spermatozoa, such as those of Crustacea and Nematoidea, which have not the filamentous appendage, and are motionless. The vibratory motion of filamentous spermatozoa bears some resem blance to that of some kinds of fine cilia, and is the most apparent indication of the active state of their vitality.f
It is now ascertained beyond doubt that in a number of animals the spermatozoa come into direct contact with the yolk substance and embryogerm, or with the internal con tents of the ovum. The actual entrance of the spermatozoa into the ovum has been observed in Mammalia, Batrachia, Osseous Fishes, Insects, Nematoid Worms, some Blollusca, and Echinodermata ; and there have been ascertained circumstances regarding the ova of other animals which warrant the inference that the spermatozoa enter the ovurn in many more than those in which the phenomenon has already been actually ob served. After long continued doubt and much discussion of this point, physiologists are therefore now generally agreed that in all instances a direct action of the sperma tozoa in substance on the contents of the egg is necessary to fecundation. The manner of access of the spermatozoa to the interior of the ovum is probably various in different animals. In a few, as Trematode and Cestoid Entozoa, the sperm is mixed with the contents of the ovum, viz., the germinal vesicle and yolk, at the time when these are brought together from the separate organs in which they are formed : in some, as the Nematoid Worms, and probably also in some other animals, the sperm comes in contact with the ovuin previous to the formation of an en veloping membrane ; in a third set it seems probable that, as in Lumbricus, and perhaps in some Mollusca and Ilirudinea, the vitelline membrane which had existed at an earlier period is dissolved or removed previous to fecundation, and the ovum or yolk substance and germ are thus left directly exposed to the action of the spermatozoa, which in Lum bricus have been observed in great numbers penetrating the substance of the yolk.
In the majority of animals, however, the sperm only reaches the ovum at a later stage of its formation, when it is already covered by the vitelline membrane or other envelopes, and through these coverings, therefore, the spermatozoa must pass to gain access to the yolk and germ. In a certain number of ani mals the vitelline or enveloping membrane appears to be quite entire and closed on all sides, so that, as in Mammalia, in which Martin Barry was the first in 1843 to perceive with certainty the entrance or the spermatozoa into the ovum, these bodies must in some way, not yet fully known, pass through the consistent wall of the enclosing membrane ; but in other animals, as first discovered by J. 'Miler, a special aperture or perforation of the egg-covering exists, apparently destined to allow of the more rapid entrance of the spermatic bodies. This micropyle apparatus, sometimes consisting of one, and at others of a number of apertures, has now been observed in several Echinodermata, in Acephalous Mollusca, in all Insects, and in Osseous Fishes ; and it is more than probable that it exists in a considerable number of other animals in which it has not yet been detected. But still, making due allowance for the probable extension of discovery in this direction, the care and accuracy with which the micropyle apparatus has since its first discovery been sought for without success in Mammalia and some other animals, in which, had it been present, it could scarcely have escaped so careful a scrutiny, warrant the belief that in a certain number of animals the spermatozoa do actually penetrate the apparently entire egg covering.