Sex-Reversal and Hereditary Constitution

In the case of Amphibia, fishes and birds, the conditions re quired for complete sex-reversal are readily met. In the Amphib ians and birds it has been shown that it is possible experimentally to masculinise a female and to feminise a male by appropriate sex-gland implantation. Male and female accessory sexual ap paratuses and external sex-organs are very similar, the sexual differences commonly being nothing more than differences in the degree of development of common structures, or, in the case of such as are developed from different rudiments, the set appro priate to the alternative sex becoming completely atrophied. All that is required is that one kind of sex-gland tissue shall be replaced by the alternative kind. In the case of the Amphibia if, as seems the case, at the end of the breeding season the sex glands are physiologically exhausted, if the differentiated tissues undergo complete involution so that a new proliferation of germinal epithelium is required for the provision of gametes for the following breeding season, or if only a portion of the pri mordial germ cells develop each year, then the mode of differentia tion of each season's crop can be determined by the impress of varying environmental agencies, if these are of such a nature as to disturb sufficiently the general metabolism of the individual. Such would appear to be the explanation of Champy's results. Surgical removal of the gonad will lead to the same result if following gonadectomy the general metabolism of the individual is influenced by special feeding (Harms, 1923).

It is to be emphasised that, although the processes of sex-differ entiation are reversible, the chromosome constitution of an indi vidual is not thereby affected. The form and function of the gamete are not determined by its chromosome content : they are determined by the structure and function of the gonad in which the gamete is elaborated. In forms in which sex-reversal is usual it seems that the physiological state established by the hereditary constitution is readily overridden through the environment. Re versal is an adaptive response to a changing environment—the individual is a female when it may be and a male when it must.

THE

The sex-ratio is the numerical proportion of the sexes within a group. In biological literature it is commonly recorded as the number of males per hundred females : in biometrical papers it is expressed as the percentage of males in the data examined. A third and somewhat elaborate method is one that shows the pro portion of males as a decimal of unity. In any bisexual species there must be a sex-ratio at all times after the sex-determining mechanism has operated. For purposes of discussion it is con venient to take conception, birth, and maturity as the three salient points in the life-history of the individual at which to compute the sex-ratio of the species, and the proportions which obtain at these three stages are known as the primary, the sec ondary, and the tertiary sex-ratio respectively. Of these the sec

ondary sex-ratio has received most attention. In most animals only the tertiary sex-ratio is known.

The Tertiary Sex-ratio.

The tertiary sex-ratio will be identical with the secondary unless during the period birth– maturity sex-reversal commonly occurs or unless a sexually selec tive mortality has been operating. Any difference between the sec ondary and tertiary ratios can simply be the result of a selective postnatal mortality and can serve as an indication of the relative postnatal survival value of the sexes.

The only material available for examination is the human. The returns of the Registrar-General for 1913 reveal the follow ing facts. In the age groups o-5 years the sex-ratio of infantile mortality is 113.4; in the 5-10 group it is ioo.7; in the 1 o-15 group it is 93.3. This period, io-15, is, in fact, the only one dur ing which more females than males die, and this is held to be the result of the exhaustion of puberty in the female and the incidence of tuberculosis (Schultz, 1918, and Stewart, 1910-1911). From 15 years on the sex-ratio of mortality rises steadily to the age of 50, when a slight decline sets in. This excess of male mortality is probably mainly due to occupational stress. The effect of this se lective mortality is seen in the swing of the sex-ratio of the popu lation; 104 :Ioo at birth, it is reduced to 102 : ioo at the end of the first year, and to io 1.5 :10o at the end of the second. In the third to fifth years it falls to 101.3:10o. From 5-10 it becomes 99.9 ioo; from io-15 it becomes 94.2:10o. During the 15-2o age period there is a rise following upon the increased mortality of females during the 10-15 age group. From this point on there is a continuous drop, with the exception of a slight rise between 4o and 5o during which period there is an increased mortality of females from reproductive disorders during the period 40-45 years. In old age (85 years) the sex-ratio is only 55.2:100.

It is impossible as yet to define the causes of this postnatal sex ually selective mortality. The action of semi-lethal factors (see on), differences in occupational risks, and such like, cannot ex plain all the facts, and all that can be said at present is that for reasons as yet unknown the male exhibits an inherently inferior resistance to the stresses of the acts of living.

The Secondary Sex-ratio.

The secondary sex-ratio in a group will be the same as the primary unless during the period con ception–birth (hatching) sex-reversal is common, or else a sex ually selective mortality among male and female embryos op erates. Any differences between primary and secondary ratio will suggest that the sexes possess different prenatal survival values.