Chapter 7

SEX IN TERMS OF INTERNAL SECRETIONS

Continuity of germplasm; The sex chromosome; The internal secretions and the sex complex; The male and the female type of body; How removal of sex glands affects body type; Sex determination; Share of egg and sperm in heredity; Nature of sex--sexual selection of little importance; The four main types of secretory systems; Sex and sex-instincts of rats modified by surgery; Dual basis for sex; Opposite-sex basis in every individual; The Free-Martin cattle; Partial reversal of sex in man.

In Chapter I, the "immortality" of the protoplasm in the germ cells of higher animals, as well as in simpler forms without distinct bodies, was mentioned. In these higher animals this protoplasm is known as _germplasm_, that in body cells as _somatoplasm_.

All that is really meant by "immortality" in a germplasm is continuity. That is, while an individual may consist of a colony of millions of cells, all of these spring from one cell and it a germ cell--the fertilized ovum. This first divides to form a new group of germ cells, which are within the embryo or new body when it begins to develop, and so on through indefinite generations. Thus the germ cells in an individual living to-day are the lineal descendants, by simple division, of the germ cells in his ancestors as many generations, or thousands of generations, ago as we care to imagine. All the complicated body specializations and sex phenomena may be regarded as super-imposed upon or grouped around this succession of germ cells, continuous by simple division.

The type of body in each generation depends upon this germplasm, but the germplasm is not supposed to be in any way modified by the body (except, of course, that severe enough accidents might damage it). Thus we resemble our parents only because the germplasm which directs our development is a split-off portion of the same continuous line of germ cells which directed their development, that of their fathers, and so on back. This now universally accepted theory is called the "continuity of the germplasm."

It will be seen at once that this seems to preclude any possibility of a child's inheriting from its parents anything which these did not themselves inherit. The bodies of each generation are, so to speak, mere "buds" from the continuous lines of germplasm. If we _develop_ our muscles or our musical talent, this development is of the body and dies with it, though the physical basis or capacity we ourselves inherited is still in the germplasm and is therefore passed along to our children. We may also furnish our children an environment which will stimulate their desire and lend opportunity for similar or greater advancement than our own. This is _social inheritance_, or the product of _environment_--easy to confuse with that of _heredity_ and very difficult to separate, especially in the case of mental traits.

It will likewise become clear as we proceed that there is no mechanism or relationship known to biology which could account for what is popularly termed "pre-natal influence." A developing embryo has its own circulation, so insulated from that of the mother that only a few of the most virulent and insidious disease germs can ever pass the barrier. The general health of the mother is of utmost importance to the vitality, chances of life, constitution and immunity from disease of the unborn child. Especially must she be free from diseases which may be communicated to the child either before or at the time of birth. This applies particularly to gonorrhoea, one of the most widely prevalent as well as most ancient of maladies, and syphilis, another disastrous and very common plague which is directly communicable. As to "birthmarks" and the like being directly caused by things the mother has seen or thought about, such beliefs seem to be founded on a few remarkable pure coincidences and a great deal of folk-lore.

Reproduction in its simplest form is, then, simply the division of one cell into two parts, each of which develops into a replica of the original. Division is also the first stage in reproduction in the most complicated animal bodies. To get an idea of what takes place in such a division we must remember that a cell consists of three distinct parts: (a) the protoplasm or cytoplasm, (b) the nucleus, and (c) a small body known as the centrosome which need not be discussed here.

When a cell division takes place, the nucleus breaks up into a number of thread-like portions which are known as chromosomes. There are supposed to be 24 pairs, or 48, in the human cell. All the evidence indicates that these chromosomes carry the "factors" in inheritance which produces the characters or characteristics of the individual body.

In mitosis or ordinary cell division, these chromosomes split lengthwise, so that the new cells always have the same number as the original one. When the germ-cells of the male and female make the division which marks the first step in reproduction, however, the process is different. Half the chromatin material passes into each of the two cells formed. This is called _maturation_, or the maturation division, and the new cells have only half the original number of chromosomes. Each of these divides again by mitosis (the chromosomes splitting lengthwise), the half or haploid number remaining. The result is the _gametes_ (literally "marrying cells"--from the Greek _gamé_, signifying marriage). Those from the male are called sperms or spermatozoa and those from the female eggs or ova. (The divisions to form ova present certain complications which need not be taken up in detail here.) Of the 24 chromosomes in each sperm or egg we are here concerned with only one, known as the sex chromosome because, in addition to transmitting other characteristics, it determines the sex of the new individual.

Neither the ovum nor the spermatozoon (the human race is referred to) is capable alone of developing into a new individual. They must join in the process known as fertilization. The sperm penetrates the egg (within the body of the female) and the 24 chromosomes from each source, male and female, are re-grouped in a new nucleus with 48 chromosomes--the full number.

The chances are half and half that the new individual thus begun will be of a given sex, for the following reason: There is a structural difference, supposed to be fundamentally chemical, between the cells of a female body and those of a male. The result is that the gametes (sperm and eggs) they respectively produce in maturation are not exactly alike as to chromosome composition. All the eggs contain what is known as the "X" type of sex chromosome. But only half the male sperm have this type--in the other half is found one of somewhat different type, known as "Y." (This, again, is for the human species--in some animals the mechanism and arrangement is somewhat different.) If a sperm and egg both carrying the X-type of chromosome unite in fertilization, the resulting embryo is a female. If an X unites with a Y, the result is a male. Since each combination happens in about half the cases, the race is about half male and half female.

Thus sex is inherited, like other characters, by the action of the chromatin material of the cell nucleus. As Goldschmidt[1] remarks, this theory of the visible mechanism of sex distribution "is to-day so far proven that the demonstration stands on the level of an experimental proof in physics or chemistry." But why and how does this nuclear material determine sex? In other words, what is the nature of the process of differentiation into male and female which it sets in motion?

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.

Each of these sex types, male and female, varies somewhat within itself, as is true of everything living. The two are not so far apart but that they may overlap occasionally in some details. For instance, some women are larger than are some men--have lower pitched voices, etc. The whole bodily metabolism, resting as it does upon a chemical complex, is obviously more like the male average in some women than it is in others, and _vice versa_. But the average physical make-up which we find associated with the male and female sex glands, respectively, is distinctive in each case, and a vast majority of individuals of each sex conform nearly enough to the average so that classification presents no difficulty.

The extreme as well as the average body types existing in the presence of the respective types of sex-glands are different. For example, we find an occasional hen with male spurs, comb or wattles, though she is a normal female in every other respect, and lays eggs.[4] But we never find a functional female (which lays eggs) with _all_ the typical characteristics of the male body. Body variation can go only so far in the presence of each type of primary sexuality (i.e., sex-glands).

The bodily peculiarities of each sex, as distinguished from the sex-glands or gonads themselves, are known as _secondary_ sex characters. To put our statement in the paragraph above in another form, the primary and secondary sex do not always correspond in all details. We shall find as we proceed that our original tentative definition of sex as the ability to produce in the one case sperm, in the other eggs, is sometimes difficult to apply. What shall we say of a sterile individual, which produces neither? The problem is especially embarrassing when the primary and secondary sex do not correspond, as is sometimes the case.

Even in a fully grown animal, to remove or exchange the sex glands (by surgery) modifies the bodily type. One of the most familiar cases of removal is the gelding or desexed horse. His appearance and disposition are different from the stallion, especially if the operation takes place while he is very young. The reason he resembles a normal male in many respects is simply that sexuality in such highly-organized mammals is of the whole body, not of the sex-glands or organs alone.

Suppose this horse was desexed at two years old. Nearly three years had elapsed since he was a fertilized egg. During the eleven months or so he spent within his mother, he developed a very complicated body. Beginning as a male, with a male-type metabolism (that is, as the result of a union between an X and a Y chromosome, not two X's), all his glands, as well as the body structures they control, developed in its presence. Not only the sex glands, but the liver, suprarenals, thyroid--the whole body in fact--became adjusted to the male type. He had long before birth what we call a male sex complex. Complex it is, but it is, nevertheless, easy enough to imagine its nature for illustrative purposes. It is simply all the endocrine or hormone-producing organs organized into a balanced chemical system--adjusted to each other.

When the horse had had this body and this gland system for nearly three years (eleven months within his mother's body and twenty-four outside), it had become pretty well organised and fixed. When a single chemical element (the hormones from the sex-glands) was withdrawn, the system (thus stereotyped in a developed body and glands) was modified but not entirely upset. The sex complex remained male in many respects. It had come to depend upon the other chemical plants, so to speak, quite as much as upon the sex glands. The later the castration is performed--the more fixed the body and gland type has become--the closer the horse will resemble a normal male. Much laboratory experimentation now goes to show that some accident while this horse was still a fertilized egg or a very small embryo might have upset this male type of body chemistry--perhaps even caused him to develop into a female instead, if it took place early enough. This is well illustrated by the so-called "Free-Martin" cattle, to be described later.

For a long time a controversy raged as to whether sex is determined at the time of fertilization, before or after. Biologists now generally prefer to say that a fertilized egg is "predisposed" to maleness or femaleness, instead of "determined." The word "determined" suggests finality, whereas the embryo appears to have in the beginning only a strong tendency or predisposition toward one sex type or the other. It is now quite commonly believed that this predisposition arises from the _quantity_ rather than the quality or kind of factors in the chemical impetus in the nuclei of the conjugating gametes. A later chapter will be devoted to explaining the quantitative theory of sex.

Hence the modern theory of "sex determination" has become:

1. That the chemical factors which give rise to one sex or the other are present in the sperm and ovum _before_ fertilization;

2. That a tendency or predisposition toward maleness or femaleness arises at the time of fertilization, depending upon which type of sperm unites with the uniform type of egg (in some species the sperm is uniform while the egg varies);

3. That this predisposition is:

a. Weaker at first, before it builds up much of a body and gland system to fix it;

b. Increasingly stronger as the new body becomes organized and developed;