Love: A Treatise on the Science of Sex-attraction / for the use of Physicians and Students of Medical Jurisprudence

PART IV.
PHYSIOLOGY OF SEX

CHAPTER VI
CELL-DIVISION, MATURITION, IMPREGNATION

The main function of the genital glands is the production of germ-cells, the spermatozoa in the male and the ova in the female. This production is effected by means of cell-division. The division may be direct as found in the fissiparous reproduction of the amoeba. As soon as the cell reaches a certain size, it divides itself into two cells. The more complicated form of cell-division is the indirect division called “Mitosis” or “Karyokynesis.”

As shown before the cell consists of two main parts, the smaller part called the nucleus and the larger portion called the cytoplasm. The nucleus shows in its interior a linin net and deeply staining granules. Because of this quality they are called “chromatin.” The cytoplasm in the close proximity of the nucleus contains minute granules, situated either singly or in pairs which are called “centrosomes.”

In the indirect division the nucleus is divided by a complicated process. Simultaneously the cytoplasm and the membrane are also cleft in two parts. In the typical cell division two parallel series of changes occur nearly simultaneously, one affecting the nucleus, the other the cytoplasm. The chromatin which is usually (Fig. A, Cut XXVIII) in form of scattered granules, arranged along the linin network, becomes aggregated together in certain definite areas (Fig. B), forming usually a convoluted thread or skein. This skein appears either in form of a long filament or divided up into a series of segments called “chromosomes” (Fig. C). The number of chromosomes is constant for each species of plant and animal. Thus, in the common mouse these chromosomes are twenty-four in number, in the onion sixteen, in the sea-urchin eighteen, etc. The number is always an even one. By this time the nuclear membrane has disappeared, and the chromosomes appear usually as a collection of bands lying free in the cytoplasm (Fig. D).

At the same time, another series of changes has gone through the centrosome and the cytoplasm in the cell-body. The centrosome assumes an ellipsoid form, constricts transversely into a dumb-bell-shaped figure (Fig. B), and divides into two daughter centrosomes. Around each of them is gradually developed a stellate figure composed of a countless number of delicate fibrils, radiating out in all directions from the centrosome as a centre. The entire constellation is called “aster.” The two asters grow in size progressively as the two centrosomes
move apart toward the poles of the cell (Fig. C). Between the two asters a spindle-shaped system of delicate fibrils appears stretching from one aster to the other which is called “central-spindle” (Fig. D). The two asters together with the central-spindle represent the “Amphiaster”.

At this point the centrosomes or the asters and the chromosomes begin to work together. A system of fibrils grows out from each aster which attach themselves to the individual chromosomes. The latter bent into U-shaped loops arrange themselves in a circle around the centre of the spindle and form the “equatorial plate” (Fig. E).

The chromosomes are now longitudinally split, and the halves move toward the poles as if drawn by the mantle fibres (Fig. F). The loops are still connected with each other by these connecting mantle fibres (Fig. G and H).

The new U-shaped loops form now new skeins at each pole (Fig. I). The chromatin granules now separate along the thread of the linin network, and the new nuclear membranes are formed (Fig. K).

Simultaneously with the forming of the two daughter nuclei the cell-body constricts across the middle of the somewhat elongated cell. The constriction increases until a complete division in the equatorial plane of the spindle has taken place. The result is the formation of two separate daughter cells (Fig. L).

Maturition.—This mode of cell division is made use of by the organism in the maturing of the germ cells into gametes. Before the conjugation of the ovum with the spermatozoön takes place, most ova extrude two polar bodies in the following way. Twice a division of the ovum takes place, each time into two quite unequal parts. The smaller, called polar bodies, remain near the periphery of the egg cell and are extruded later on. The same phenomenon is observed in the spermatocytes. The immature germ cell can neither impregnate nor become impregnated respectively, until, by a twice repeated cell division, a part of the nuclear substance has been cast off. Only after this has been effected does the germ cell become a gamete (a marriageable cell, from the Greek word γαμέω, I marry), i. e., a cell capable to impregnate or being impregnated.

This maturition of the germ cell has hitherto been veiled in a mystery, which did not admit any plausible explanation. Why is the loss of half of the chromosomes necessary? Nowadays it has been accepted by most of the biologists that maturition stands in close connection with Mendel’s law of segregation.

The learned monk Mendel, in his convent garden, made several important discoveries concerning the heredity of living organisms. He first discovered the quality which he called “unit characters”. Unit characters are, in the first place, the characteristics of the species, such as the number of the fingers. Unit characters are further characteristic of the sex, as the beard in the male and the breast in the female. Lastly there are the individual unit characters, as black hair or blue eyes. Unit characters are controlled by determiners which are either dominant or recessive. Unit characters, for this reason, do not blend. The color of the eyes, for example, is a unit character, and black is the dominant color. Hence when a black animal or plant is crossed with a white one, the hybrid is always black. The black type predominates in the influence upon the hybrid, while the type of white exercises the lesser influence.

The second phenomenon Mendel discovered is that of segregation. By segregation is understood the separation of opposite determiners. Every unripe ovum or spermatocyte in the hybrid plant or animal, contains, for example, white and black determiners, but the ripe ovum or spermatozoön contains only one kind of determiner, either white or black, that is, during the ripening of the germ cell into the gamete one kind of the determiners has been eliminated.

Segregation, therefore, means that the gamete, or sex cell after maturition, has either dominant or recessive determiners, never both. Segregation thus affects the purity of the gametes. The matured ovum and spermatozoön are always pure, even in the hybrid plants or animals. Accordingly, when a spermatozoön, with a white determiner impregnates an ovum with a white determiner, although both originated in black hybrids, the zygote, or the impregnated cell, will be a pure white. When a black ovum is impregnated by a black spermatozoön, the zygote will be a pure black. If the ovum is black and the spermatozoön is white or the ovum is white and the spermatozoön is black, the zygote will also be black, but a black hybrid.

Black + black = black, 25% pure.
Black + white = black, 25% hybrid.
White + black = black, 25% hybrid.
White + white = white, 25% pure.

Thus, when a black bean e. c. is crossed with a white bean, all the offspring in the first generation will be black but hybrids. When two of these hybrids are then crossed, the dominant color black will be represented by 75 per cent. of the offspring and the recessive color white by 25 per cent., i. e., the offspring in the second generation, although both parents are black, will be three-fourths black and one-fourth white. For the gamete of two hybrid parents is always pure. The zygote of two hybrid parents is either pure or hybrid. When the two gametes are similar, the zygote is pure and is called homozygous, when the two gametes are dissimilar, the zygote is heterozygous. Hybrids, by the faculty of segregation, produce in the first generation of hybridism, fifty per cent. homozygous zygotes (25% pure whites and 25% pure blacks) and fifty per cent. heterozygous zygotes (all hybrid blacks). Hence the apparent paradoxical phenomenon that when a pure blond is married to a pure brunette, all the children are brunette, while when both the parents are brunette, but hybrids, the offspring will be twenty-five per cent. blond and only seventy-five per cent. brunette. The blond being recessive can only be pure, while the brunette being dominant, may be either pure or hybrid.

This law, that the determiner in the protoplasm of the parent cell, or rather in the nucleus of the cell, always fixes the character of the progeny, holds good only of the unit character. Many individual characters are fluctuations and play no part in Mendelian heredity. Bodily modifications, resulting from environing conditions, are not Mendelized. Most of the human traits Mendelize, such as stature, span, size of head, shades of color of hair and eyes, hair curliness, pulse rate, digestion and the psychic traits, such as determination, cheerfulness, alertness, resistance to fatigue. Some anomalies also depend upon the determiners and Mendelize, such as colorblindness,
ness, night blindness, albinoism, brachydactylism (only two finger joints instead of three), syndactylism, polydactylism, keratosis, hemophilia, cataract, deaf-mutism, imbecility, Hutchinson’s chorea, epilepsy, and some forms of insanity.

The Mendelian law of segregation has somewhat lifted the veil in which maturition was wrapped. Every germ cell, being a product of two parent cells, is in one way or other a hybrid. By casting off half of the chromosomes during its maturing, it becomes pure. The gamete is thus always pure and ready for impregnation.

Impregnation.—The process of impregnation is about the same in most many-celled animals. As soon as the head of a single spermatozoön enters the egg-cytoplasm, a new membrane is formed around the ovum which effectually prevents the entrance of any other spermatozoa. The head and the middle piece penetrate now into the egg, the tail usually remaining imbedded in the membrane where it soon degenerates.

A few moments after the spermatozoön has entered the egg, a system of radiation appears around the middle piece which develops into an aster, surrounding the centrosome of the sperm cell (Cut 29, Fig. B.).

The sperm nucleus now increases in size, and its chromatin changes into a reticulate form. Sperm aster and sperm nucleus, the aster preceding, now move toward the egg nucleus. As the nuclei approach each other the sperm nucleus increases still more in size, until both become almost of the same size (Fig. C.).

The chromatin network of each nucleus now breaks up into a number of chromosomes and the nuclei come into contact and fuse together. The centrosome, together with its aster, divide now into two parts, and the two daughter centrosomes move apart to the opposite poles of the ovum, and the typical amphi-aster of cell division, as above described, is formed (Fig. D.).

The nuclear membranes now disappear and the chromosomes are drawn together into the equatorial plate where each splits longitudinally. The halves are drawn by the mantle fibrils toward the opposite poles where they are transformed into two daughter nuclei (Fig. E.). In the meantime the cytoplasm has also divided. The result are two new cells. This process of division is repeated continuously in each of the resulting generations of cells. From the mass of cells thus formed develops the new organism.