Sex Experiments Of Nature
( Originally Published 1940 )
AFTER these preliminary attempts to find our bearings, we shall start from the very origin and consider the fertilised ovum as it lies in the uterus, in its most primitive embryonic condition, before it is attached to the uterine wall. At this period it derives a scanty nourishment from the maternal serum in which it floats. Not till later, at the end of the first three months, when the original store of energy possessed by the fertilised ovum is coming to an end, do a few blood-vessels grow from the embryo towards the uterine wall, where they ramify to form the placenta (afterbirth). Through this, by osmosis, the blood of the embryo obtains nourishment from and gives up its waste products to the blood of the mother. The offspring is now no longer known as the embryo but as the foetus and lives as a parasite on the maternal circulation until its birth. Thereafter it has an independent existence, feeds on other organisms, and grows, at first swiftly, but with gradually decreasing energy until puberty.
The first or embryonic stage is a very primitive one. The gill-clefts are beginning to form the face. The whole body is still open in the medial line anteriorly. Two primordial kidneys subserve the scanty urinary function. Four small eminences appear about the third week, which will soon develop into legs and arms. All external evidence of sexual differentiation is still lacking.
At this stage one cannot discern even any internal sexual differentiation. Development proceeds quite systematically. Every process of growth takes place by normal cell division and all the different tissues develop pari passu without mutual interference. When the embryo has developed to a certain point and the primordial kidney (Wolffian body) is completely formed, a new phenomenon occurs. There now appears at each side of the lumbar column, a thickening of the epithelial layer which lines the interior of the body cavity. These gradually develop as small growths or tumours. They are the first rudiments of the bilateral reproductive tumours. The first evidence of this change to be seen under the microscope is a tendency of the epithelial cells and the underlying connective Tissue to invade each other. This increases until the epithelial cells become separated into small aggregations surrounded by connective tissue.
Here, too, the segregation of these cell-masses has most far-reaching consequences—it is the beginning of the whole sexual life. In the ordinary course of events these cells would have become differentiated to form a surface epithelium, and, when old and degenerated, would have been cast off as dead cells. Now, however, that they are buried deep in other tissues they retain their original embryonic characters, no matter how much they are multiplied by cell division. When at last the time of their final cell division comes, they will reach the surface as young cells.
These isolated masses of epithelial cells may develop further in one of two different ways. This is the starting point of a divergence which becomes an actual sexual differentiation at puberty. In some embryos there is always one cell in each mass which attains a larger size and will eventually develop into a mature egg-cell. Such an embryo will eventually become a female. The formation of such egg-cell rudiments continues until the end of the second year of extra-uterine life.
In other embryos, however, i.e., in those which are to become males, all the cells of each aggregation undergo multiplication and form a large number of tiny cells which become arranged close together round a small central cavity. These tumours continue to grow and come to invade two neighboring glandular organs which are situated on either side of the vertebral column. The latter are the primordial kidneys (Wolffian bodies) which by this time are functioning. They are invaded, and the tiny cavities, which have meantime elongated to form tubules, join with the microscopic tubules of the primordial kidney. The two elements now form a compound organ known henceforward as the testis.
Later on, in the adult male, only the efferent sexual canal is derived from the Wolffian body, while the reproductive cells are furnished by the other part of the testis which corresponds to the original reproductive tumour. The two Wolffian bodies cease to function as secretors of urine and are replaced by two new, active, fully developed kidneys. But even in the adult testis, the typical glandular structure remains so obvious that, at first sight, one would think it was only a typical gland.
So on each side the epithelial protuberance develops into a mass which bulges out more and more and finally separates off from its base. In this way, the two testes lose their support posteriorly and come to sink down anteriorly. They are now only suspended by their efferent ducts which are, so to speak, their pedicles. They sink downwards a graceful curve on either side of the urinary bladder until they reach first the inguinal region and finally the scrotum (vide chapter 1) .
So later, after puberty, when the sperm-cells are voided, they follow the original urinary passage of the Wolffian body which at first ascends steeply, until the two sperm-ducts approach each other from right and left to join the unpaired urethra deep in the abdominal cavity.
In the same way two reproductive masses of the female type become separated from their bases, but remain attached by a few strands of connective tissue, so that the two ovaries do not sink down nearly so far as do the testicles. Furthermore, there is no coalescence with a glandular organ, so an ovary is considerably smaller and flatter than a testicle, of simpler shape, just like a tumour. For in embryos which undergo female development, the Wolffian bodies and their efferent ducts disappear, leaving only a few vestiges. For this reason, when in the course of time the egg-cells become ripe, they will not be so fortunate as to find an efferent duct ready for them, as do the sperm-cells. After puberty, each time an egg-cell ripens, it must, like an abscess, force its way straight through the ovarian tissue by destruction of the surrounding cells. It reaches the surface of the ovary, bulges out-wards, and, leaving traces of hemorrhage and scar tissue behind, it becomes free and falls into the abdominal cavity. And what follows?
Fortunately, in every embryo, there is on each side not only the Wolffian body, with its duct, but also another canal. This opens exteriorly at the lower pole of the body and remains open at its inner end within the abdominal cavity.
Finally, we come to the external "genitals" which we may better call the external efferent-duct-system of the embryonic Wolffian body and the rudimentary pronephros, for only after puberty will this system be used for sexual purposes.
Both of the above-mentioned efferent canals lead to the lower pole of the body of the embryo, where their orifice forms a small protrusion called (in anticipation) the genital eminence. Later on in embryonic life, when the body cavity is closed and the genital eminence has become a small protuberance, it is still difficult to decide from this small papilla, whether it will develop into a clitoris with a short urethra or into a penis. Likewise, a pair of folds between the thighs may grow together to form a scrotum or remain separated to form two labia majora. Not before the tenth week after fertilisation, i.e., towards the close of the embryonic period, does the future sexual differentiation begin to be evident externally. In the male, the urethra becomes lengthened and is joined on either side, by the "seminal" canals. Long before birth, these organs take on their permanent form. Although for many years the whole apparatus will continue to grow pari passu with other organs, yet the genital function will remain in abeyance, and meanwhile only the urinary system will be active.