To begin with, we must give some account of the difference between the cells of male and female origin, an unlikeness capable of producing the two distinct types of gametes, not only in external appearance, but in chromosome makeup as well.  It is due to the presence in the bodies of higher animals of a considerable number of glands, such as the thyroid in the throat and the suprarenals just over the kidneys.  These pour secretions into the blood stream, determining its chemical quality and hence how it will influence the growth or, when grown, the stable structure of other organs and cells.  They are called endocrine glands or organs, and their chemical contributions to the blood are known as hormones.

Sometimes those which do nothing but furnish these secretions are spoken of as “ductless glands,” from their structure.  The hormones (endocrine or internal secretions) do not come from the ductless glands alone—­but the liver and other glands contribute hormones to the blood stream, in addition to their other functions.  Some authorities think that “every cell in the body is an organ of internal secretion",[2] and that thus each influences all the others.  The sex glands are especially important as endocrine organs; in fact the somatic cells are organized around the germ cells, as pointed out above.  Hence the sex glands may be considered as the keys or central factors in the two chemical systems, the male and the female type.

These various hormones or chemical controllers in the blood interact in a nicely balanced chemical system.  Taken as a whole this is often called the “secretory balance” or “internal secretory balance.”  This balance is literally the key to the sex differences we see, because it lies back of them; i.e., there are two general types of secretory balance, one for males and one for females.  Not only are the secretions from the male and the female sex glands themselves quite unlike, but the whole chemical system, balance or “complex” involved is different.  Because of this dual basis for metabolism or body chemistry, centering in the sex glands, no organ or cell in a male body can be exactly like the corresponding one in a female body.

In highly organized forms like the mammals (including man), sex is linked up with all the internal secretions, and hence is of the whole body.[3] As Bell [2, p.5] states it:  “We must focus at one and the same time the two essential processes of life—­the individual metabolism and the reproductive metabolism.  They are interdependent.  Indeed, the individual metabolism is the reproductive metabolism.”

Here, then, is the reason men have larger, differently formed bodies than women—­why they have heavier bones, tend to grow beards, and so on.  The sex glands are only part of what we may call a well-organized chemical laboratory, delivering various products to the blood, but always in the same general proportions for a given sex.  The ingredients which come from the sex glands are also qualitatively different, as has been repeatedly proved by injections and otherwise.