The genetic and the operative evidence relating to secondary sexual characters

by Thomas Hunt Morgan

Another hermaphrodite (Atwood’s black) had an infantile oviduct and an ovotestis. A second bird, too, had an ovotestis—mostly testis—as well as a rather large oviduct. Collections of luteal cells are described between the tubules of the testicular portion. If, as suggested by the Sebright cases, these cells tend to suppress the female plumage, their presence here in excess might at least be made to account for the female part of the plumage of this bird. Comparing the last two birds (that showed active sex-behavior as males) with the best of the Holland birds, Boring and Pearl point out that the active sex behavior of the two former can not be due to “interstitial cells that are absent in these but present to a slight extent in the former.” They then add” ...though the differences can not be laid to the lutear cells, as they are present in all three.” That the relative amounts of the latter or their activity might still be accountable for the difference would not seem entirely excluded from the evidence so far as it is given.

A fourth hermaphrodite (Dexter’s) laid 12 eggs and had a large coiled oviduct. There was present “a large, lobulated reproductive organ on the left,” which proved to be an ovotestis. Several ovarian tumors were present and there was testicular tissue.

It is fairly evident, then, that four of these birds described by Boring and Pearl were females with abnormal ovaries. The incomplete development of the latter, or their abnormal condition due to tumors, may sufficiently explain the occurrence of male secondary sexual characters. That these tumors affect, to different degrees, such characters is expected from what is shown by imperfectly spayed females of normal breeds.

There are a few statements in the summary of this paper that call for comment. The statement that the “development of comb, spurs, and wattles does not stand in direct quantitative relation to the sex of the gonad,” appears to be only intended as a statement of fact based on the author’s observation. But in what sense is there an expectation that they should stand in such relation beyond the obvious fact that in the cock the comb and wattles are larger than in the hen, and that spurs are generally present only on the cock. But if the expression “sex of the gonad” implies the germ-cells it is not at all certain that there is any expectation of a quantitative relation, and there is some probability at least that other cells than the sex-cells are involved in the development of combs, wattles, and possibly spurs. A castrated cock has a small comb resembling that of the female bird. On the other hand, removal of the ovary sometimes leads to an increase in the comb and wattles. Here we have, to say the least, a paradoxical situation, for the result looks superficially as though something in the ovary keeps down the hen’s comb, while something in the testes keeps up the cock’s comb, yet when the ovary is removed the hen develops a cock’s comb; when the testes are removed the cock develops a hen’s comb. The real meaning is, I think, that the genetic complex for femaleness (one Z or else ZW) stands in itself for a full-sized comb, while the genetic complex for maleness (two Z’s) stands in itself for small comb.

Boring and Pearl state that “body-shape and carriage have a genetic relation to the sex of the gonad.” This statement means, I think, that the amount of testicular matter present stands in some direct relation to the shape of the body and carriage of the male. Castration, both of the normal cock and the Sebright, seems to change the carriage somewhat and perhaps the shape. Both lose something of the peculiar attitude of the male, but I have not been able to my own satisfaction to analyze what this means. As has been pointed out, and as the pictures show, the castrated Sebright changes his attitude, but whether this is a change due to his new contour, or to a new balance resulting from a large tail, or to a let-down resulting principally from effects on the nervous system, is difficult to determine. The same statements apply in part to the castrated cock of ordinary breeds, but not to the same degree, since the change after castration, in feathering and in carriage at least, is slight.

The conclusions that the “amount of lutear cells or pigment (?) is in precise correlation with the degree of external somatic femaleness exhibited by the individual” is of especial interest in connection with the Sebright evidence. It is difficult, however, to gather from the body of the paper what the absolute amount of luteal cells is that is present, for even in some of the more male-like birds with an ovotestis the description leads one to suppose that there may be as much luteal material present as in some of the more female birds with infantile ovaries or cystic tumors.

Pearl and Curtis (1909) described “a case of incomplete hermaphroditism” in a Barred Plymouth Rock fowl. Externally the bird looked like a hen, but “the head and neck resembled these parts in the cockerel,” especially the comb and wattles. The bird was never seen to tread a hen, nor did it ever crow normally. An ovary and oviduct were found on the left side, the former no larger than that of a laying hen after removal of the large yolks. No eggs were visible on its surface. On the right side a testis (9 mm. by 6 mm.) and vas deferens were present. No eggs were found in the ovary, and it gave every indication of being in a degenerating condition, with no eggs or egg follicles in it. The testis had no “normal seminiferous tubules”, but indications of cellular rods were present. The organ is in all probability a degenerating testis.

A Leghorn 2 years old has been described by Shattuck and Seligmann (1906) that had the full-developed comb and wattles of the cock, but the former drooped slightly to one side as in the hen. Well-developed spurs were present. The plumage was mainly female, with neck-hackles moderately developed, and with “saddle-hackles” practically absent. The tail, though not typically female, lacks sickle feathers. The bird excited no notice from other birds of either sex. A large left oviduct and the distal end of a right oviduct were present. Two vasa deferentia were also present. In the left side a flattened sex-gland (3 cm. high) was found, made up of testicular tubules. Two small ova were found in its posterior end. The right gonad was also tubular (testis).

The occurrence of real testicular tissue in one of the Holland birds and in three others described by Boring and Pearl, as well as in one described by Pearl and Curtis, and in another by Shattuck and Seligmann calls for special comment, since the presence of both testicular and ovarian tissue in the same bird is the essence of hermaphroditism. In general there are two ways of looking at such a result. Either the sex-determining factors have been changed so that in one part of the body, where the reproductive organs are laid down, one condition can prevail, in other parts other conditions; or a mixup of the sex chromosomes has taken place. Until we get some more evidence concerning such cases it is useless to speculate, although the former view might seem the most probable of the two if the Holland birds of Herr Houwink’s flock were in a high degree true hermaphrodites.

But in fact three of the four described by Boring and Pearl were due to tumors of the ovary, which, if they suppress the normal development of this organ, would be expected to call forth the appearance of the secondary sexual characters of the cock. If the likelihood of developing a tumor were inherited, the frequent occurrence of hen-feathered birds in this flock would be explained. However, one true hermaphrodite in 4 birds is surprisingly high for a chance result, since hermaphrodite birds are very rare.