August i8, 192 i] 



NATURE 



783 



Early castration in mammals prevents the normal 

 ■development of the visible characters, and results 

 in the assumption even of female secondary char- 

 acters by male birds. Early and successful trans- 

 plantation of the heterologous gonad makes either 

 sex assume, to a more or less complete degree, the 

 characters of the other sex (Steinach, Goodale, 

 etc.). We might term this the production of hor- 

 monic intersexuality, but, of course, changes 

 appear only in those organs which are able physio- 

 logically to change under the influence of hor- 

 mones, irrespective of the genetic constitution in 

 regard to sex. But the methods which have to 

 be used exclude a complete experiment in inter- 

 sexuality, where the entire body, including sex- 

 glands, ducts, etc., must react. Fortunately, 

 Nature has performed such an experiment for us, 

 as the recent analysis of the case of the "free- 

 xnartin," which we owe to the work of Keller, 

 Tandler, and Lillie, has revealed. Among twin 

 calves, cases of normal male and female are very 

 rare. If both are not of the same sex, in most 

 cases a normal male is accompanied by an ab- 

 normal hermaphrodite female, the freemartin. It 

 is now known that this freemartin is a typical 

 case of hormonic intersexuality. The authors 

 quoted above have been able to show independ- 

 ently that in this case — but in this case alone — 

 an anastomosis between the blood-vessels of the 

 twins occurs, so that the same blood flows 

 through both. In the male partner the testis, 

 with its interstitial tissue, develops first, and 

 before the ovary of the female has reached the 

 stage of endocrine function. So the female comes 

 under the influence of the male hormones, the 

 ovary stops diff^erentiation, and all the sex-char- 

 acters develop in the male direction. The result 

 is the freemartin, a calf with female external sex- 

 organs, almost male sexual ducts, and a sex-gland 

 containing sperm tubules which are incapable of 

 spermatogenesis. Most interesting corroboration 

 of this interpretation has recently been furnished 

 in Lillie 's laboratory by Minoura, who was able 

 to produce hormonic intersexuality experimentally 

 by transplanting gonads into developing chickens' 

 «ggs. 



If we compare this case of true hormonic inter- 

 sexuality with the zygotic intersexuality of the 

 moths, we see at once that the "turning point" 

 from which sexual differentiation changes in 

 the intersexual moth corresponds exactly to 

 the moment when the male hormones are 

 poured into the blood of the female in 

 the case of the freemartin. Comparing the 

 facts carefully, we feel justified, therefore, in 

 giving the following answer to our former ques- 

 tion : What is this reaction which is accelerated 

 bv the action of the sex-substances with a velo- 

 city proportional to their concentration? The re- 

 action is the production of the specific hormones 

 of sexual differentiation. In insects this occurs 

 in every cell of the body as an irreversible conse- 

 quence of the combination in fertilisation. In the 

 higher vertebrates the reaction becomes more or 

 NO. 2703, VOL. 107] 



less centralised within the interstitial tissue of the 

 sex-glands. 



That this solution of the problem of sex comes 

 near the truth is rendered probable by the ease 

 with which even the most complicated sexual 

 phenomena fall in line with the theory. The ques- 

 tions of parthenogenesis and sex, sex-mosaics or 

 gynandromorphs, sexual polymorphism, inherit- 

 ance of secondary sex-characters, and the different 

 types of hermaphroditism, all find simple solu- 

 tions, or, at least, appear capable of such. This 

 may be demonstrated in the interesting case of 

 the Gephyrean worm Bonellia, well known for 

 its extreme sexual dimorphism, the male being a 

 rudimentary microscopic worm which lives as a 

 parasite in the oviduct of the large female. 

 Baltzer made the discovery that part of the larvae, 

 developed from fertilised eggs, become attached 

 to the proboscis of an adult female, and live 

 there for some time in a semi-parasitic way before 

 developing into males. Larvae, however, which 

 undergo development without the parasitic stage 

 remain for some time undifferentiated, and then 

 develop into females. If larvae which are fixed 

 to the proboscis of a female are removed after 

 a shorter or longer period, intersexes of different 

 type are produced. Let us now suppose that we 

 could devise an experiment to prove directly the 

 correctness of the quantitative view of sex-deter- 

 mination as represented in the above graph. We 

 might perform it successfully by finding a method 

 of accelerating or retarding the rate of differentia- 

 tion without influencing the rate of the production 

 of the sex-hormones. In the event of success we 

 ought to be able to shift the point of intersection 

 of the F and M curves back into the time when 

 differentiation was still in progress. The result 

 would be intersexuality. It seems that Bonellia 

 is able to perform this experiment by means of 

 the excretion of her proboscis. The F and M 

 curves of the larvae seem to have such a relation 

 that the male hormones are being produced quickly, 

 and the female hormones slowly. The normal 

 rate of difi^erentiation is slow — so slow that sexual 

 differentiation begins only when the phase of 

 action of the male hormones has passed, and 

 females are produced exclusively. The secretion 

 of the proboscis, however, accelerates the rate of 

 differentiation in a way analogous to the action of 

 the thyroid in accelerating metamorphosis in 

 amphibians. In the case of parasitism of the 

 larvae, therefore, differentiation takes place during 

 the phase of action of the male hormones. Inter- 

 ruption of the influence of the secretion naturally 

 causes intersexuality. Finally, we may state that 

 recently we were successful to a certain extent 

 in imitating this experiment with moths. By em- 

 ploying low temperatures we could put back the 

 turning point for females of pure races of the 

 gipsy-moth and thus produce intersexuality. 



Ever since genetics assumed its modern form 

 the problem of sex has been closely linked with 

 the general problem of heredity. The Mendelian 

 study of sex formed part of the general study of 



