It will be plain from this that the germ of the future sex must be sought in the first cell-segmentation of an ovum. As soon as some of the cells derived from the primary protoplasm of the ovum have developed themselves into genital cells, the other elements which have originated from the same ovum are in such a manner conditioned that, in the latter stages of their vitality, they adapt themselves, and, in short, adapt the properties inherent in all the cells, to the sex of the individual. According, as the ovum is male or female, so are also the cells which originate from it either all male or all female.
It will be seen that not only do different cells for the different sexes develop themselves out of an ovum, but that also, at the same time with these, a peculiarity reveals itself in the other cells, in accordance with which the sexually different organisms exhibit a difference in their vital capacities, and take also different forms. The distinction between male and female characteristics appears to be determined before the fecundation of the ovum. The formation of the ova in the ovary, and their further development, seem, however, not to be independent of external influences. It is possible that upon these circumstances depend the number of ova contained in the ovary. But, apart from the question of quantity, it is possible that many characteristics might so affect the quality of the ovum, as to exercise an influence over its capacity for fecundation. We may here mention an experiment which was made with the ova of a rabbit, from which it was quite clear that the capacity of an ovum for fecundation was immediately diminished when the surrounding elements attached to the ovule, in consequence of the density of their investing substance, offered a resistance to penetration by the spermatozoa. (Schenk.) The penetration of a spermatozoon into the protoplasm of the ovum becomes possible only when, in consequence of the movements of the spermatozoon, the cells of the surface of the ovum can be thrust aside. This is facilitated when the investing substance is considerably relaxed, as is the case when the ovum is ripe. Other circumstances, also, which can in some cases be easily detected, may prove detrimental to fecundation and development. Indeed, they can even exercise an influence over the sex which is to be developed out of the ovum. Bee-masters (F. Gerstung) have frequently shown that the food exercises a striking influence upon the formation of sex in the ova. (v. Berlepsch.)
All evidence goes to support the view that such external influences as would favorably affect the separate sexual individual might also promote the production of one sex or the other. In Hensen’s valuable work on generation, a number of instances are adduced, gathered from various authors, which make it clear that the nutrition of the parents, apart from any question of race, is capable of exercising an influence upon the sex of the children. (Ploss.) In plants which produce separate male and female blossoms (Lenkhart), the male blossoms are more numerous when the temperature is relatively high, whilst in shaded places and damp soils a greater number of female individuals will be observed.
Facts which might assist to explain the origin of sex have been sought after from very early times, and have been also placed in very different lights. The result on every occasion, when this subject has been discussed, has been always a wide difference of opinion. People have, in consequence, been induced to fall back upon theories of different sorts, theories which have for varying periods, sometimes long, sometimes short, been accepted as of some assistance towards a scientific explanation. In all the theories which have been propounded, the sex has been regarded as already determined in the ovum, or else the origin of the sex has been assigned to some early stage of the development.
The earliest statements extend back into the ages of myth and fable, in consequence of which any exact comparison of them is not an easy task. All the different manuals which deal with the present question touch upon these early views, and for this reason I am unwilling entirely to ignore them here. I shall, accordingly, select a few of the more important for mention.
The reproductive glands of the two sexes were supposed to contain generative matter distributed in such a way, on the right and left, that the right ovary and the right testicle contained the generative secretions for the production of the males, and the left ovary and left testicle those which produced females. It is immediately evident that, according to this theory, it was impossible to exercise any influence over the sex of the future individual. This primitive theory is ever cropping up anew, always to be again rejected. Of various other theories of the same kind, only those deserve any attention which rest upon some basis of fact. Accordingly, recourse has been had to statistics, and an attempt has been made to reach, from the figures which they furnish, some certainty respecting which sex was the more numerous, and what should be concluded to be the cause of the greater prevalence of the one sex or the other. The fact was, however, apparently overlooked that the available statistics, though in many respects of the highest scientific value, could be of real significance only when the numbers were gathered from widely distributed peoples amongst whom there was none of that wandering about the world which characterizes modern society.
I am at the same time unwilling to omit data, resting upon numbers which have been gathered from statistics, and are not without value for the determination of many important questions.
Ploss has in this way shown that in favorable years, when food was cheap, the births showed an excess of girls. Under unfavorable circumstances, more male individuals were born.
A comparison of statistics, however, soon led to another theory, which culminated in this result, that in all countries an excess of male individuals was born.