Sex determination

The determination of the sex of an individual, with regard to both the primary sex—i.e., whether the ovaries or the testes develop—and the various secondary sexual characteristics may be rigorously controlled from the start of development or may be subject to later influences of a hormonal or environmental nature. However this may be, in order to appreciate the action of the control systems, the point of departure is that animals were primitively hermaphrodite, that during early stages of evolution every individual probably possessed both male and female gonads. Differentiation into separate sexes, each possessing male or female gonads but not both at the same time, is a device to ensure cross-fertilization of eggs, whether this is accomplished by having the two types of sexual gland mature at different stages of the growth of the individual, as in some shrimp and others, or whether by the production of two distinct types of individuals, as in most species of animals. This point of view is important because the question ceases to be how testes are caused to develop in the male organism and ovaries in the female but how, in a potentially double-sexed organism, the development of one or the other sex is suppressed. That such is the case is seen as clearly as anywhere in the human condition itself. Neither sex is completely male or female. Females have functional, well-developed mammary glands. Males also have mammary glands, undeveloped and nonfunctional although equipped with nipples. Males have a penis for delivering sperm, but females have a small, nonfunctional equivalent—the clitoris. These are secondary sexual features, to be sure, but the difference between the sexes is in the degree of their development, not a matter of absolute presence or absence.

The basis for this is seen in the very beginnings of the development of the reproductive system, in frog, mouse, and man alike. In the young embryo a pair of gonads develop that are indifferent or neutral, showing no indication whether they are destined to develop into testes or ovaries. There are also two different duct systems, one of which can develop into the female system of oviducts and related apparatus and the other into the male sperm duct system. As development of the embryo proceeds, either the male or the female reproductive tissue differentiates in the originally neutral gonad of the mammal.

In the frog and other lower vertebrate animals, the picture is even clearer. The original gonad consists of an outer layer of cells and an inner core of cells. If the individual is to be a male, the central tissue grows at the expense of the outer layer. If it is to be a female, the outer tissue grows at the expense of the central core tissue. If both should grow, which is a possibility although a rare occurrence, the individual will be a hermaphrodite. Anything that influences the direction taken therefore may be said to determine sex.