According to Pflüger, the epithelium is wanting in such very young follicles. In these frogs after their metamorphosis the testicle consists of tubes, with multinuclear epithelia, and is easily distinguishable from the always large ovaries of the tadpoles. By different concentrations of semen, Pflüger attempted to influence the proportion between the male and female sexes.
According to Pflüger, there are normally found amongst frogs in a state of nature 36.3 per cent. males, and 63.7 per cent. females. With thinner or thicker semen, the average number can be altered. With thin semen Pflüger obtained 27.3 males and 72.7 females per cent. With concentrated semen he obtained 39.4 males, and 60.6 females. Pflüger carried out some other experiments, and came to the conclusion that the concentration of the semen or the extract from the testicles exercised either a very small influence or no influence at all upon the sex. When he took an average result from all his experiments, he found that out of 806 frogs which he raised, 288 were males. Whilst the normal proportion of the males developed freely under natural conditions was 36.3 per cent., that reached by experiment was 35.7 per cent. It should be here remarked that among the tadpoles many are found whose sex is not yet determined. They are in a hermaphrodite condition, out of which they develop into either males or females.
Robin, the well-known French histologist, made the question of the origin of sex the subject of an extensive inquiry. His investigations start from the following point:—In warm climates the whole quantity of blood in the inhabitants is less than in temperate zones. The process of respiration in the inhabitants of warm climates is also not so free as in the case of those who inhabit temperate or cold zones.
From this it would seem that some process connected with nutrition, and with the passage of nourishment into the blood, is the cause of the number of male births being greater in the cold zones than it is in the temperate zones, or in the inhabited regions lying nearer to the equator. It would follow from these considerations, that if the women were subjected to such a régime as would materially affect their respiration and the quality of their blood, more boys than girls, or the contrary, might be bred. If so, breathing an atmosphere containing more oxygen, with a corresponding diet, would be the right receipt for producing in the woman such a basis that in the course of development the male generative organs (which Robin considers the anatomically more perfectly developed) might be evolved instead of the female. According to Robin, the male sexual apparatus in comparison with that of the female is provided with the more perfectly developed character. Robin further insists that strong men will beget more male individuals. Further, that a woman who indulges in sexual intercourse somewhat seldom has female children; and that voluptuous women, who are fruitful, generally bear boys. A number of experiments with domesticated mammalia are adduced in support of this view.
Hegar teaches that in the case of a merely rudimentary development of the germinal gland either sex is developed.
With the views of Robin may be connected also other suggestions regarding the food of the parents. These have been tried both with men and animals. But we shall not here go further into them. Nor shall we mention the different kinds of food or drink which have been employed, whether by men or women, to produce a greater sexual activity.
The eminent naturalist Born, of Breslau, made a long series of experiments, which are of the highest interest in reference to the doctrine of the origin of sex. It is easy to fertilize frogs’ eggs artificially. The ripe eggs are taken directly from the female, and the testicles of the male rubbed with water. This fluid, which now contains spermatozoa, serves to fertilize the eggs. Spallenzani had already undertaken artificial fructification. Born observed, during his study of the course of development, that the effect of his breeding as regarded sex was to produce 95 per cent. of females. This number is evidently so remarkable that it ought to secure particular attention. No such extreme contrast between the numbers of males and females is to be found amongst the frogs that develop freely under natural circumstances. It seemed to Born that his result was to be referred to insufficient nourishment, and that the tadpoles, being somewhat unfavorably circumstanced, had not been able to attain the development of the stronger sex. In this experiment on the evolution of sex it appeared that not only was there an excessive production of females, but that the other organs of the embryo and its whole constitution could be modified by means of nutrition. It is also to be observed respecting this most interesting experiment that many of the tadpoles perished of hunger. Now, the number of still-born males of the human species very much exceeds the number of still-born girls. The mortality amongst males is so great that the average is from 136 to 140 still-born boys to 100 still-born girls. An attempt has been made (Pflüger) to explain this phenomenon in the human subject on the ground that the tenacity of life in the female sex in the period of embryonal existence exceeded that of the male. Consequently, boys would more easily perish during development than girls. If this observation made with respect to human beings were applied to the tadpoles, it might also explain the high percentage which the females showed among the frogs. Very likely the male tadpoles possessed less capacity of resistance; or, in other words, were less tenacious of life than the females. It will be understood that this view would apply only as a partial explanation of the facts set forth by Born.