The human embryo, like all vertebrate embryos, is furnished at first with the basal parts of both sexes. It is usual, therefore, to suppose that we have descended from a hermaphrodite form—one in which the organs of both sexes were combined. This cannot be regarded as a good explanation, for although in a few rare cases an imperfect sexual gland has been found, which showed some resemblance to the structure of a testicle in one part and to an ovary in another, there is not a single case of a true testicle and a true ovary having been found in the same person. A true human hermaphrodite has not been seen. The explanation which best accounts for the presence of both male and female ducts in the embryo is the fact that the male has to be the father of daughters as well as of sons; for that reason he must have a representation of the parts necessary to the one sex as well as of the other. Similarly the mother must be capable of handing on sexual characters to her sons as well as to her daughters. Hence both sexes are necessarily supplied with the parts proper to each sex.
This statement is also true of the breasts ; rudiments of these are present in both sexes at birth. When the human embryo is only a month old a ridge on the skin (the mammary ridge) is seen to extend along each side of its body from the armpits to the groins. Some months later the parts of these ridges which lie on the upper part of the thorax undergo a development and form the two mammae or breasts, while the rest of the ridge atrophies and disappears. The breasts undergo a slight development before, and immediately after birth, and then remain in abeyance until marked changes occur in the sexual organs at puberty. At that age they remain small in the male, but in the female they grow rapidly, and take on their characteristic size and form.
One of the most important recent advances in our knowledge of the sexual glands is the discovery that they are double in their structure and function. They are composed not only of germinal tissue, which gives origin to the reproductive elements, but also of another tissue which is glandular in nature and supplies a substance to the various parts of the body by means of the circulating blood. On some parts of the body this substance has a most powerful influence, and stimulates growth. The secretion which issues from the ovary at puberty acts upon the latent breasts, and calls forth their growth in the female; the secretion of the sexual glands of the male has no such influence. There are numerous proofs of the truth of this statement. I need not relate the facts which medical men are well aware of; those who are familiar with unsexed domestic animals will need no further proof of the influence of the testicles in moulding the form of the body. At the end of each pregnancy a secretion derived from the genital parts of the mother, which acts on the breasts, leads them to increase in size and yield a secretion of milk. These facts are extremely important to us because they reveal the secret and yet simple manner in which nature works out the physical characters of our bodies.
The breasts must be reckoned as parts— secondary parts or characters—of our sexual system. The reader already knows that milk-giving glands appeared with the evolution of mammals. The thoracic position of the milk glands is also an old feature of the stock from which mankind arose, for in all monkeys and anthropoids we find only two mammae, which occupy the same position as in man. In the ape embryo, as in the human, a mammary ridge is formed along each side of the body. In many mammals, such as the sow and bitch, mammae are formed on the whole length of the mammary ridge—from armpit to groin. It occasionally happens in men and women that besides the usual pair of breasts, others may be formed just below or just above the normal breast glands. Rudiments of supernumerary mammae and nipples are not rare ; vestiges may be seen in one out of every twenty men or women. We may infer from these facts that the stock from which the ape-like mammals descended was one with breasts arranged along the length of the body as in the pig. The supernumerary nipples and mammae which occasionally occur in man are revivals of characters belonging to a remote stage in the evolution of the mammalian stock.
At birth the boy and girl are much alike in appearance. It is true the boy has usually a stouter and heavier body and a larger head than the girl, yet it is only from the deeper parts that the sex can be told with certainty. At birth and even before birth, the sexual glands are influencing the development of the body. Professor Arthur Thomson, of Oxford, noted that in the foetus of five months the pelvic bones of the two sexes were slightly different in shape. It is at puberty that the glandular influence becomes most marked. We have already seen how rapid the increase of stature is in girls from the twelfth to the fourteenth year, and in boys between fourteen and sixteen. It is at the period of puberty that the breasts of the female develop ; at the same time the pelvis takes on its roomy character. The breadth in the lower part of the body becomes a marked feature in woman. The tops of the thigh-bones which articulate at the sides of the pelvis are pushed outwards and become relatively further apart than in the male. Hence when walking, women have to make rather a greater effort than men, for with each step the weight of her body has to be transferred across her wide pelvis from one thigh to the other. At puberty, too, the type of breathing changes; respiration becomes centred more in the upper than in the lower part of the chest. The thorax changes in form. The use of corsets tends to emphasize, even to a degree of distortion, the normal prominence of the upper part of the thorax of women, and at the same time, by constricting the waist, to bring into prominence the lateral projection of the pelvic part of the body. In all races and in both sexes, fashion is often the excuse for exaggerating the natural characters of the body.