SEX-REVERSAL AND HEREDITARY CONSTITUTION Sex-reversal can be the result of the overriding of the heredi tary constitution by agencies which sufficiently disturb the general physiological conditions of the zygote at some stage or other of its development. It can result from a disturbance of the physio logical condition within the ovum before fertilisation. The work of Hertwig, Kushakevitch and Witschi has shown that delayed fertilization and also the exposure of frogs' eggs before fertilization to high temperature (27° C) lead to a profound disturbance of sex-ratio. A male frog was permitted to fertilise half the eggs of a female and then was removed, to be replaced after an interval to fertilise the remainder. After an interval of 89 hours none but male offspring were obtained. This result is not due to selective fertilisation, to a sexually selective mortality among the embryos, or to the abnormal extrusion of an X-chromosome during the maturation of the eggs. The correct interpretation of the results would seem to be that some 5o per cent of the eggs were fertilised by X-chromosome-bearing spermatozoa, females (XX) being produced, but that the conditions of the experiment were such as to transform these into functional males, the sex-chromosome constitution of the zygote being over ridden by the effects of delayed fertilisation upon the metabolism of the egg. The results obtained by Mrsie on the effect of over ripeness upon trout eggs are probably to be explained in a similar fashion.
The observations of Adler (192o), who has shown that the thyroids of individuals from these late fertilised eggs are markedly hypertrophied, would seem to be of significance. Adler suggests that in these individuals the thyroid comes into action earlier than does the gonad and so affects the internal environment that the gonads, when they do differentiate, become testes.
These observations are closely in line with those of Whitman (1919) and of Riddle, (1912, 1916) upon the pigeon. It was found by Whitman that the matings of birds belonging to the Columbidae with species of two distinct zoological families of birds resulted in the production of male offspring only, and that females alone were obtained from the eggs of doves which had been forced to lay excessively and at an abnormally rapid rate.
Riddle carried these observations further and was able to show that the eggs that yield males can be distinguished from those which yield females, that maleness is associated with eggs of smaller size, higher water content, and less stored energy, and that the production of all males or of all females was associated with the production of eggs of one or of the other type. He was able to dismiss the possibility of selective fertilisation and of differential maturation, and was driven to the conclusion that the conditions of the experiments were such as to induce sex-reversal in the egg itself (1914, It is a simple matter to interpret these results in terms of a metabolic theory of sex as elaborated by Riddle. Delayed fertili sation implies an increased metabolic rate in the egg and a high metabolic rate implies maleness. Desiccation implies a decreased metabolic rate and femaleness. The hypertrophy of the thyroid implies an increased metabolism and an internal environment of maleness. The production of offspring all of one sex by matings of wide crosses is to be interpreted as the result of the pooling of hereditary factors which in their action lead to the estab lishment of one kind or the other of metabolic level in the zygote.
In this connection it should be noted that Guyer's (1909) data on species hybrids among birds show that there is a decided excess of males in the F1 generation. Riddle (1916) recorded an excess of females in the cross Streptopelia risoria X S. alba (doves) under certain conditions and concluded that this excess was the result of a transformation of some of the males. It has been shown, however, that this conclusion is not justified, for the cross involved a sex-linked character and of the hybrids the males are dark, the females white in colour, and examination of the date for sex and also for colour shows that the only possible explanation of the excess of females is that which postulates that the conditions of the experiment were such as to cause the X chromosome to pass into a polar body at the time of the re duction divisions more often than to remain in the egg.