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Then as each somatic cell is descended without segregation from the fertilised ovum, we may suppose that the presence of the sex-chromosomes in the somatic cells themselves in some way determines whether male or female characters shall develop, without the aid of any hormones from the gonads.
The males give out masses of sperm, and the females discharge ova in such quantity that many of them stick to the fibrils about their mouths. Both kinds of cells pass first into the mantle-cavity after the opening of the gonads, proceed through the gill-clefts into the branchial gut, and are discharged from this through the mouth. The ova are simply round cells.
But they have been also found to hold secretory cells not concerned with the making of the reproductive corpuscles, but, as all the evidence indicates, with the manufacture of an internal secretion. These interstitial cells form the interstitial gland. A classic example of a gland of internal secretion lodged in the interstices of a gland of external secretion is thus furnished by the gonads.
At the same time there are some somatic sex-characters, e.g. in insects and birds, which do not appear to be correlated with changes in the gonads, and which are probably gametogenic, not somatogenic in origin. The theory of the heredity of somatogenic modifications is not in opposition to the mutation theory.
These traits appear only when the hormones occur which are present in one sex and that only when the gonads of that sex are mature. In some cases they appear only at the period of the year when reproduction takes place, disappearing again after the breeding season. After castration, the hormones being absent, all these points of contrast between the sexes fail to appear.
As regards the remaining organs of the Amphioxus, we need only mention that the gonads or sexual glands are developed very late, immediately out of the inner cell-layer of the body-cavity. For the rest, the subsequent development into the adult Amphioxus of the larva we have followed is so simple that we need not go further into it here.
But others display signs of sex differentiation that are to be traced back to an awakening interstitial gonad action. Some boys have no interest whatever in sex. Others will show an intense curiosity spontaneously, a curiosity which perhaps may be explained as a larval precocity, dependent upon the minimum of sex hormone production by the gonads.
The adaptive relation is not the only common characteristic of these somatic sexual characters. Another most important fact is not only that they are fully developed in one sex, absent or rudimentary in the other, but that their development is connected with the functional maturity and activity of the gonads.
For example, we find an occasional hen with male spurs, comb or wattles, though she is a normal female in every other respect, and lays eggs. The bodily peculiarities of each sex, as distinguished from the sex-glands or gonads themselves, are known as secondary sex characters.
These are characters or organs more or less completely limited to one sex. When we distinguish in the higher animals the generative organs or gonads on the one hand from the body or soma on the other, we see that all differences between the sexes, except the gonads, are somatic, and we may call them somatic sexual characters.
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