SCIENCE.
THE RIDDLE OF SEX.
FEw subjects have provided a more popular play- ground for amateur speculation than that of sex. Few subjects of direct human interest have yielded more readily to scientific inquiry during the past quarter of a century. Twenty years ago the factors which determine whether the embryo will develop into a male or a female remained as obscure to the biologist as to the layman. But since the late Professor Doncaster of Liverpool un- ravelled the inheritance of colour pattern in the currant moth an entirely new horizon has opened up for the scientific investigation of sex. The result of Doncaster's work (1905) and the brilliant corroborative testimony which American investigation—especially-that of Morgan's school—has added to it have now been available for several years to the educated public through his ex- tremely lucid book, The Determination of Sex (Cambridge University Press).
The essence of Doncaster's discovery—sex-linked inheritance as it is technically described—is that there are certain bodily characters which individuals of one sex the male in man-and most animals, the female in birds and moths—can only transmit to their offspring of the other sex. For instance, females of the red-eyed fruit fly Drosophila have only red-eyed offspring when mated to -white-eyed flies, while red-eyed male fruit flies mated to white-eyed mothers give only red-eyed females, the males being white-eyed. The analysis of all the results .of crossing in such a case lead to the conclusion that one- half the male reproductive elements or sperm cannot carry the material antecedent of the red eye, and since these sperms are all destined -to fertilize eggs which will become males it follows that the male individual is so constituted that it produces two sorts of sperms, one which fertilizing an egg gives rise to male offspring, the other which fertilizing an egg gives rise to female offspring.
The conception that there exists a mechanism in the re- productive cells which determines sex from the very origin of a new life has been developed on the basis of an immense mass of breeding experiments in a very wide range of animal forms ; but it has been clarified and confirmed in a quite spectacular manner by the study of the reproductive cells with the aid of the microscope, and nowadays no serious student of the biological problem of sex seriously adheres to the traditional doctrine that the sex of the individual is decided in a haphazard manner merely by forces such as nutrition incident to prenatal existence. During the past century it was established that. the tissues of animals are built up of microscopic bricks„ the cells, which increase by binary division. It is from the union of two cells—the sperm and the egg cell of the mother—that the life history of a new organism is initiated; and it is through the repeated division of this combined cell (the fertilized egg) that the new indi- vidual develops. Towards the end of the last century there were recognized in dividing cells certain minute structures known as chromosomes, which divide indi- vidually in cell division, so that the number of chromo- somes in dividing cells of any species of animal is in general constant. Van Beneden and Boveri showed -in the 'eighties that this constancy is maintained from one generation to another. The chromosomes do not divide in the penultimate cell division in the reproductive organ, leading up to the function of the ripe sperms or eggs, but are distributed so that each sperm or egg contains half the number characteristic of the species ; the relatively small number of animals and plants at that time investigated all possessed even numbers of chromosomes in their dividing tissue cells. In the opening years of our own century it began to be recognized, however, that in some animals one pair of chromosomes equally mated in one sex is repre- sented by an odd element (the " X chromosome ") in the other sex, which thus possess an uneven number of chromosomes. For example, it has been shown by the writer that the cockroach has thirty-four chromosomes in the female and thirty-three in the male. All the ripe eggs of such females will have seventeen chromosomes, but the sperms will have either sixteen or seventeen. According as an egg is fertilized by the former or the latter the fertilized egg will develop into an individual having thirty-three chromosomes in the dividing cells of its body (i.e., a male) or thirty-four chromosomes in its tissue cells (i.e., -a female). These facts precisely harmonize with the study of " sex-linked inheritance " : animals in which tile female can only transmit certain characters to offspring of the other sex (e.g., moths) have a single " X chromosome " in the female, so that one-half the eggs bear the " X chromosome " and the other half do not ; while animals like the fruit fly in which the male can transmit certain of his qualities only to his daughters have an odd " X chromosome " in the male, so that one half of the sperms—those alone which fertilize eggs which will become females—bear the " X " element.
Thus, from the standpoint of modern biology the difference between the sexes is a phenomenon which is identifiable in the microscopic structure of every cell of the body from the very act of fertilization which initiates the life history of a new individual. How, then, does it come about that sex appears to be dependent on environ- mental agencies—climate, food and the like ? For a proper answer to this question the reader will be able to turn to Professor Dakin's translation of Goldschmidt's Mechanism of Sex Determination, now being issued by Messrs. Methuen, a book which embodies practically all the important work since Doncaster's review of the subject appeared some years ago. To-day, many indis- putable instances of sex reversal are known, such as the life history of the slipper limpet, in which if the larva settles near a female it becomes first a male and later a female ; while, if it does not settle in propinquity to another individual, it passes at once into the female con- dition. An even more spectacular case is that of the worm Bonellia, in which the male is a minute degenerate parasite living inside the secretory organs of the female. Here the larva which settles alone on the sea floor becomes a female, the larva which settles on the proboscis of a female becomes a male, and the newly settled larvae of the latter class if detached from the female proboscis become hermaphrodites. There is no essential disharmony between the view that factors are inherent in the sperm or egg and the belief that factors operating from the animal's environment determine the future sex of the offspring. All inborn characters need the appropriate physiological conditions—external and internal—in order that they may express themselves. Sex is not irreversible, because an individual may have the constitution (a single " X chromosome " in the fruit fly, for example) which nor- mally goes with maleness and yet be female in bodily organization through some outward inhibition or patho- logical condition. Dr. Crew, the young and brilliant director of the new Animal Breeding Station at Edinburgh, has shown that if female frogs change over into males they beget only female offspring, presumably because all the sperms of such transformed males will possess the " X chromosome."
The most far-reaching advances towards a knowledge of how the sex chromosomes operate to produce sex differentiation comes through the work of Professor Richard Goldschmidt himself. Goldschmidt has for the last ten years studied crosses between different geo- graphical races of the Gypsy moth, which, as entomolo- gists well know, give offspring showing intermediate sex characteristics and described as " intersexes." By an extensive mass of experimental data, brilliantly analysed, Goldschmidt has clearly shown that these abnormal results of crossing are due to the relatively different efficiency of the constitutional forces which determine sex in the different races of this moth. But the most interesting result of his investigation is that he is able to describe the degree of intersexuality of these moth hybrids, so that the male or female character of any particular organ, the feathering of the feelers, the colour of the wings, &c., depends on the time at which the structure develops in the embryonic history of the individual. Following up this line, he has been driven to the con- clusion that the presence of a single or double " X chromosome " determines solely the time in development at which one or the other sex-controlling forces pre- dominate. Thus, the forces which determine sex both towards maleness and femaleness operate in both sexes, but in a fully developed male the male-determining system is dominant at the time when the sexually diffe- rentiable structures arise in development, while in the fully developed female the female-determining system dominates during this period. This conception enables one to envisage in one consistent scheme the conclusions from breeding experiment and microscopic study on the one hand, and the undoubted facts of sex reversal in many animals on the other.
Of current work following this line of inquiry some of the most striking studies are those being pursued by Dr.
Crew at the Edinburgh Animal Breeding Station, where the attempt is being made to interpret the facts of sex in birds and mammals in accordance with Gold- schmidt's conceptions. In our nearest allies, the domestic animals, we commonly distinguish those sexual charac- teristics which develop after birth—at puberty—as secondary sex characters, e.g., the plumage of the cock, antlers of the stag, beard in man, &c. It has been known from antiquity that the manifestation of these characters is related to the functional activity of the internal reproductive gland, so that removal (castra- tion) of the latter results in inhibition of one or the other set of secondary sex characters. Thus in the case of the stag, castration of the male before maturity prevents the appearance of the antlers, while in the fowl removal of the ovary causes the bird to assume the plumage and spurs of the cock. It has now been shown by the re- searches of Steinach and his school in Vienna, and of Lillie and his pupils in Chicago, that from the very first appearance of the reproductive gland in the embryo it influences subsequent development so as to encourage sexual differentiation along its own lines. By grafting ovaries into castrated male guinea-pigs and testes into females from which the ovaries had been removed, individuals showing complete reversal of sexual behaviour have resulted—an achievement not without medical possibilities. At the last meeting of the British Associa- tion Dr. Crew described his researches on the occurrence of a form of hermaphroditism or intersexuality in Pigs, goats, and (more rarely) oxen, where individuals occur from time to time with the internal gland of the male sex, the internal generative ducts of both sexes, the external organs of the female and the secondary sex characters of the male. Such cases as these, including most cases of imperfect sex differentiation which occur in man, are really males in which the male differentiating secretions have not come into operation at the right period in de- velopment. In fowls it is well known that old hens often assume characteristics of the cock. Crew has collected a remarkable series of sexually abnormal fowls culminating in one bird which, having laid fertile eggs, not only assumed the aspect, behaviour and voice of the cock, but successfully fertilized eggs of one of his own daughters with the production of fertile young. Crew finds that in any hen of a highly fecund breed seminiferous tissue makes its appearance as the ovary degenerates with age. His material suggests that the ovary normally maintains by its dominant influence the female characterization long after the female differentiating sex influence has waned in other cells of the body, so that if the ovary is destroyed by disease a measure of sex reversal ensues, and may culminate in complete transformation if the bird survives.
LANCELOT HOGBEN.
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