Baltzer has suggested on the basis of some observa­tions that the larvæ while on the proboscis of the female absorb some substance secreted by the proboscis, and this substance accelerates the further development into a male and suppresses the female tendency. If this substance from the proboscis does not reach the larvæ the tendency to become males is gradually suppressed in the majority and only a few develop into pure males or protandric hermaph­ro­dites, while the female characters are given a chance to develop. Baltzer assumes, therefore,—as it seems to us correctly—that in all larvæ the tendency for both sexual characters is present, that they are, in other words, hermaph­ro­dites, but the chance for the suppression of one and the development of the other group of characters can be influenced by certain chemical substances which the larva may take up.[190]

Giard has studied the effects of a curious form of castra­tion brought about by parasites, which is followed by a change in the sexual character of the castrated animal. The phenomenon is very striking in certain forms of crabs when they are attacked by a parasitic crustacean, Sacculina. The two sexes differ in the crab Carcinus mænas by the form of the abdomen, but when a male is attacked by the parasite its abdomen assumes the female shape. Smith observed in another crab that in such cases even the abdominal appendages of the male may be trans­formed into those of a female. The trans­forma­tion is so complete that the older observers had reached the conclusion that the parasite attacked only the females, since they overlooked the fact that the castra­tion by the parasite trans­formed the secondary sexual characters of the male into those of a female.

Giard observed that in a diœcious plant, Lychnis dioica, a parasitic fungus brings about the trans­forma­tion of the host into a hermaph­ro­dite.

G. Smith has discovered a fact which shows that chemical changes must underlie these morpho­logical trans­forma­tions of primary or secondary sexual characters. He noticed that in male crabs the presence of the parasite Sacculina changes the contents of the fatty constituents in the blood, making them equal to that of the female. Vaney and Meignon had previously shown that during the chrysalid stage the female silkworms have always more glycogen and less fat than the males. The castra­tion by parasites is paralleled by what Caullery calls the castra­tion by senility.[191] In certain birds and also in mammals at the time when the sexual glands cease to func­tion certain secondary sexual characters of the other sex make their appearance. The most common case is that certain secondary male characters appear in the old female (excep­tionally also in the young female with abnormal ovaries) (arrhenoidy). Thus old female pheasants assume the plumage of the male, and in the human female after the menopause and especially among sterile women a beard may begin to grow. The opposite phenomenon, the old male assuming female characters, is not so common. Very interesting observa­tions on changes in the plumage of castrated fowl have recently been made by Goodale.[192]

It had long been observed by cattle breeders that in the case of twins of different sex the female—the so-called free-martin—is usually sterile. F. Lillie[193] has recently discovered the cause of this interesting phenomenon. Such twins originate from two different eggs since the mother has two corpora lutea, one in each ovary. In normal single pregnancies in cattle there is never more than one corpus luteum present. The two eggs begin to develop separately in each horn of the uterus.

The rapidly elongating ova meet and fuse in the small body of the uterus at some time between the 10 mm. and the 20 mm. stage. The blood-vessels from each side then anastomose in the connecting part of the chorion; a particularly wide arterial anastomosis develops, so that either fetus can be injected from the other. The arterial circula­tion of each also overlaps the venous territory of the other, so that a constant interchange of blood takes place. If both are males or both are females no harm results from this; but if one is male and the other female, the reproductive system of the female is largely suppressed, and certain male organs even develop in the female. This is unques­tionably to be interpreted as a case of hormone action.

The reproductive system of these sterile females is for the most part of the female type, though greatly reduced. The gonad is the part most affected; so much so that most authors have interpreted it as testis.

It should be added, however, that this result cannot at present be generalized, since in the hermaph­ro­dites the specific hormones of both sexes must circulate without suppressing each other’s efficiency.

All these facts indicate that certain substances secreted by the ovaries or testes may inhibit the development of certain sexual characters of the opposite sex. When these inhibi­tions are partly or entirely removed the secondary sexual characters of the opposite sex may appear. This fact may also be interpreted as an indica­tion of a latent hermaph­ro­ditism and if this be correct the real and latent hermaph­ro­dites differ only by the degree of inhibi­tion for one sex, this inhibi­tion being lacking or less complete in the real than in the latent hermaph­ro­dite.

In the light of this conclusion the observa­tions on the regenera­tion of both ovaries and testicles which Janda observed in a hermaph­ro­ditic worm, Criodrilus lacuum,[194] is no longer so mysterious. This worm normally possesses in the segments near the head a pair of ovaries and several pairs of testes. Janda found that if the anterior parts containing the gonads of these worms are cut off a complete regenera­tion takes place, including both types of gonads, ovaries as well as testes. As a rule, more than one pair of ovaries appear in the regenerated piece. This important experi­ment shows that in a hermaph­ro­dite both types of sex organs can be produced from body cells or from latent buds resembling body cells. This phenomenon would be intelligible on the assump­tion that in the body of a hermaph­ro­dite substances circulate which favour the development of both types of sex organs, while in a diœcian animal probably only one type of sex organ would be developed; the forma­tion of the other being inhibited.