Taboo and Genetics | Page 8

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