Sex Reversal
By PROFESSOR F. A. E. CREW, M.D., D.Sc., PH.D. (Director of the Department, of Animal Genetics, University of Edinburgh.) THE biologist, acquainted with the affairs of a multi- tude of different forms of life, knows that it is no t at all uncommon for an animal to change its sex, for a female to become a male, and vice versa ; but to the lay- man, whose knowledge of living things is restricted more or less to man and to his domesticated animals, such a transformation must seem impossible.
To him every individual is either a male or else a female, the one being distinguishable from the other by clear-cut differences in structure and behaviour ; he knows that if one is born a male one stays a male. It is the case that occasionally in the morning papers one reads of the start- ling experience of someone who, after having been looked upon as a female for twenty years and more has, after marriage, been found to be a man. But these are not instances of sex reversal at all : they are cases 'of abnormal development in the male. Interesting as sex reversal in man would be to the biologist, and embarrassing as it would- be to the individual concerned, it is to be doubted that it has ever occurred. It is difficult to see how it could happen. The sex- equipment of an individual would need to be completely remodelled or replaced, and whilst this might be accom- plished in the foetus, when a change-over would not be recognized, there are no physical or chemical agencies known to science which can bring about this result in the case of the fully-formed individual. The processes which in their action lead to the establishment of the distin- guishing characters of the two sexes are, after a point, irreversible.
But what does not—probably cannot—happen to man may and does happen to other forms of life, either excep- tionally or else as a normal event in their life histories.
From the point of view of the biologist, the primary difference between male and female is that the male elaborates spermatozoa whereas the female elaborates ova. In man and his domesticated animals these two functions are performed by dissimilar individuals, the male and the female respectively. But, as one passes in a survey from mammal to bird, to fish, and onward toward the simpler forms of life, one finds that the criteria which guide one in one's definitions become less and less clear, and at last disappear altogether. One soon encounters the hermaphrodite, not as an exceptional monstrosity but as a normality, the individual being concerned with, and being equipped for, the production of both sper- matozoa and ova. The chief groups of such normal herma- phrodite animals are the flat worms (tape worms, liver flukes), earth worms and leeches, the higher univalve molluscs (sea-slugs, land snails and land slugs), barnacles and tunicates (e.g., the sea squirts).
.In many forms hermaphroditism does not consist in the synchronous production of both spermatozoa and ova by the same individual ; in them there is a periodic functional sex reversal. Commonly the individual first functions as a male, producing spermatozoa and fertilizing the ova of other individuals which, at this time, are functioning as females and producing ova, and, subse- quently, exchanges this function for the alternative and produces ova. In still other cases, hermaphroditism is not a permanent state, for maleness is an affair of youth and femaleness is an attribute of maturity. One of the most interesting instances of this type of serial herma- phroditism is furnished by the slipper limpet, a univalve mollusc which is the curse of the .oyster 'fisheries. A young individual attaches itself to the shell of an older individual and passes through the male phase of its existence : it behaves as a male in its relations with the individual to which it is attached and which, being older, is enjoying its female phase. As this young individual grows older it first becomes hermaphrodite and later develops the sex equipment appropriate to the female, and to its shell there becomes attached a still younger individual which, as time passes, goes through the same changes. So a chain of individuals is formed, and in its turn each member of the chain changes its sex. Such sex reversal is a normal happening in the case of the slipper limpet and several other forms, but it also takes place, though exceptionally, even in those forms in which normally the sex of the individual remains throughout life what it was at birth.
It can be said that if in a species hermaphroditism, even though this be imperfect, is to be found, then in that species sex reversal, more or less complete, is to be expected. Instances of true hermaphroditism—that is to say, the condition in which both spermatozoa and ova are produced—have been recorded in many kinds of fish, in the frog and toad, in the tortoise, in many species of birds, in the pig, the goat and in man. Indeed, it has been possible to produce hermaphroditism experi- mentally in several of these forms. It used to be thought that these instances of hermaphroditism meant nothing more than that the individual, for reasons unknown, had pursued an abnormal development, the final result of which was this condition in which both ovaries and testes were present. But nowadays such hermaphroditism is regarded rather as a stage in a dynamic process of sex reversal. For example, in the sword-tailed minnow, a favourite of the aquarium, it is not uncommon to find an individual with abnormal sex characters, and if one should examine its sex glands one would find both ovarian and testicular tissues. But, if one kept the individual alive, one would find that it was a female becoming a male. Fairly commonly an old female, after having given birth to several broods, develops the complete male. characterization ; her ovarian tissues degenerate completely, and in their place testicular tissues develop. The oviduct becomes transformed intc a sperm duct, and her hitherto short tail grows until it assumes the dimensions of that of the male. A female frog or toad can become, following atrophy of her ovaries, completely changed in adult life, and after having been the mother of large families can then become the father of others, equally large. At the head end of the testes of a male toad there is a peculiar body known as Bidder's organ, after the man who first described it. If the testes are removed surgically, this Bidder's organ, in the space of two or three years, becomes an ovary, and the former male lays eggs which may be fertilized by a male functioning as such. It is to be expected that any agency—atrophy or disease, for example—which obliterates the testes of the male toad without affecting other structures, can lead to a complete change of sex. He will remain a male only so long as he retains his testes.
In the female of the domestic fowl there is but one ovary,- for the sex gland on the right side of the body does not develop. But if through disease or by surgical operation this functional ovary is removed, then the sex gland on the right pursues its development. It does not become an ovary, however, but assumes the structure of an imperfect testis. It would seem, therefore, that every hen can only remain a hen so long as she retains a functional ovary. If this is removed, then inevitably she begins to develop those characters which are the property of the male.
There is a common thread running through all the woven fabric of this story of sex-change. Every individual is endowed with the potentialities of both sexes, it can develop the characters of either ; but, in the higher forms, there are innate and opposing forces which swing the mode of development in one direction or the other. The actual direction is determined by the balance which exists between these forces ; if the male-determining irripulses are greater than the female-determining, then the individual will become a male. Recent advances in biochemistry have provided reason for thinking that these forces are chemical in nature. If it should so happen that during that portion of the life of the indi- vidual when the sex equipment is developing, or at any other time when this is capable of responding to the stimulus of these sex-determining substances, there should be a reversal in their relative potencies, then sex reversal may occur. In the lower forms of life the action of these substances is influenced profoundly by the physico-chemical agencies of the environment, and so it is that among these sex reversal is common, even habitual, with increasing age or with the swing of the seasons.
Previous page
