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Here they are nourished by the other cells and develop into true reproductive elements, eggs and spermatozoa. Fertilization, that is, the union of egg and spermatozoon, or mainly of their nuclei, takes place; and the fertilized egg develops into a new organism.
Boveri, who has further extended our knowledge of the process, considers that the spermatozoon removes an inhibitory influence preventing the commencement of development in the ovum; the spermatozoon replaces a portion of the ovum which has already undergone degeneration, so that the object of conjugation is chiefly to effect the union of the properties of two cells in one, sexual fertilization achieving a division of labor with reciprocal inhibition; the two cells have renounced their original faculty of separate development in order to attain a fusion of qualities and thus render possible that production of new forms and qualities which has involved the progress of the organized world.
According to the ovists, the ovum needed merely the stimulation of the spermatozoon to cause its contained individual to undergo development, while the animalculists looked upon the spermatozoon as the essential embryo container, the ovum serving merely as a suitable food supply or growing place.
But what is sex but the difference between ovum and spermatozoon, between megagamete and microgamete? The theory then asserts that an individual developed from a cell formed by the union of male and female gametes is entirely incapable of producing female gametes again. Every zygote after conjugation or fertilisation may be said to be bisexual or hermaphrodite.
We are here in presence of a vestige of ancient theological dogma, and there can be little doubt that, on the theoretical side at all events, the "imprescriptible right" of the embryo will go the same way as the "imprescriptible right" of the spermatozöon. Both rights are indeed "imprescriptible."
The spermatozoon, which sets in motion the evolutionary process of the embryonic life, is one of the smallest cells of the organism; and it is only a small part of the spermatozoon which really takes part in the operation. But these are only superficial differences. Digging beneath them, we think, a deeper difference would be found.
This investigator found that there were 48 chromosomes in the female cell, 47 in the male; after the reduction divisions the unfertilised ova had 24 chromosomes, half the spermatids 24 and half 23, so that sex is determined in man by the spermatozoon. We may mention a few other in animals, referring the reader for a fuller account to the works cited.
Previously to this process the other sexual cell, the spermatozoon, or male reproductive cell, has been undergoing a somewhat similar process. It contains cell substance, a nucleus with chromosomes, and a centrosome, the number of chromosomes, as shown later, being however only half that normal for the ordinary cells of the animals.
We are entitled to ask why the artificial stimulus, or the entry of the spermatozoon, produces the effects which it is claimed to produce instead of any one of some score of other effects which it might conceivably have produced.
Sexual reproduction is quite a distinct matter. Here, in all these cases, what is required is the detachment of two portions of the parental organisms, which portions we know as the egg or the spermatozoon. In plants it is the ovule and the pollen-grain, as in the flowering plants, or the ovule and the antherozooid, as in the flowerless.
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