Sex determination and the physicochemical basis of heredity

Wilson, in The Cell in Development and Inheritance (1896), noted that the primordial germ-cells are at first exactly alike in the two sexes. He judged that, with a few exceptions, they are neither male nor female. Their change into ova or spermatozoa was, in his view, a reaction to an external stimulus, not an inherent predisposition. Most observations, he wrote, pointed to the character of the food, so that sex was a problem of nutrition¹. He cited Mrs. Treat, who found that starved caterpillars gave more male adults and highly fed ones gave more females¹. He concluded that sex as such is not inherited, only the capacity to become either². He could not say whether one causal element was common to all cases².

Loeb, in The Dynamics of Living Matter (1906), reported an embryologist's hypothesis that the mother's diet could fix the sex of the offspring. He gave Delage's objection. In about 30 per cent of cases twins have different sex, which diet of the mother could not allow³. Loeb added that twins from one egg always share a sex. In the same passage he judged attempts to explain heredity on a purely morphological or cytological basis acceptable. The condition was that such structures work only through their physical and chemical properties⁴.

In The Mechanistic Conception of Life (1912), Loeb said that McClung's paper had solved the problem of sex determination, at least in its essential feature. He traced the steps. Henking found two kinds of spermatozoa in an insect, one with a nucleolus and one without. Montgomery then showed that this body was an accessory chromosome. McClung first linked it to sex⁵. Loeb then described the case Wilson held true for many insects and Guyer for human beings. There are two kinds of spermatozoa and one kind of egg⁶. Loeb favoured the chromosome account in 1912. In 1916 he still suggested that outside conditions might overcome the influence of the chromosomes.

In The Organism as a Whole (1916), Loeb treated parthenogenetic females as a test. Such an egg normally carries one X chromosome. If the reduction division is omitted it has two. Loeb said this had been shown for certain cases and may be true generally. He took these cases to confirm the link between X chromosomes and sex⁷. He also suggested that sex chromosomes may favour formation of a specific internal secretion. If so, modifying secretions by outside conditions might overcome the chromosomes' influence⁸. Rádl (1930) accepted that sex follows Mendelian rules like any other character. He held that this did nothing to help understand the whole phenomenon of sex⁹.