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The reproductive cells also undergo division and increase in number, and when they separate from the new individual and unite in fertilisation they still possess all the determinants of the fertilised ovum from which they are descended. Heredity thus continues from gamete to gamete, not from zygote to soma, and then from soma to gamete.
All the cells of the body, assuming that somatic segregation does not occur, must possess the same chromosomes as the zygote from which it developed, and whether the sex chromosomes are XX or XY or X, there must be at any rate one chromosome bearing the factor for milk glands.
The fact that castration does affect, in many cases very profoundly, somatic characters confined to one sex, proves that Mendelian conceptions, however true up to a certain point, are by no means the whole truth about heredity and development. For it is the essence of Mendelism as of Weismannism that not only sex but all other congenital characters are determined in the fertilised ovum or zygote.
The remarkable fact is that from this fertilised ovum or zygote is developed usually an individual of one sex or the other, male or female, other cases being comparatively exceptional, although each act of fertilisation is the union of the two sexes together.
It might be supposed that the attached base of the dorsal fin is unable to extend forward because the eye on the edge of the head is in the way, but if the metamorphosis is arrested, why should the fin grow forward in a free projection? I have suggested that the explanation here is that in the zygote the primordia of a normal body and a reversed head have been united together.
Reversed specimens occasionally occur in many species of Pleuronectidae, and if the determinants for a reversed head and a normal body were united in one zygote, the curious abnormality observed might be the result.
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.
On the view that both sexes and the somatic sex-characters of both sexes are present in each zygote, and that the actual sex is due to dominance, we must conclude that the male primary and secondary characters are dominant on one side, and the female on the other, and it is evident that hormones diffusing throughout the body cannot determine the development of somatic sexual characters here.
It does not follow that the primary sex-character or the somatic characters are exclusive in either sex. We may suppose that the zygote contains both sexes, one or other of which is dominant, and that dominance of one primary sex involves dominance of the corresponding sexual characters.
Instead of saying that the zygote composed of ovum and spermatozoon is incapable of giving rise in the male to ova, or in the female to sperms, we should hold that the gametocytes ultimately give rise to ova or to sperms according to the metabolic processes set up and maintained in them through their successive cell-divisions under the influence of the double or single X chromosome.
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