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

CHAPTER II
EVOLUTION OF THE GENITAL SYSTEM

A. The Indifferent Stage

Before the last described stage has been reached, there has developed simultaneously with the other organs a set of organs, known under the name of the genito-urinary system, which deserves here our special attention.

The urinary secretion is effected throughout the animal kingdom by three systems: the pronephros or the head-kidney; mesonephros, or primitive kidney, or Wolffian body, “Urniere”; and metanephros, or true kidney. The pronephros must be regarded as the phylogenetically oldest part, since only traces of it are found in the human embryo. Here in the earlier stages of embryonic development, the Wolffian body is the organ for the urinary secretion.

The Wolffian body, or mesonephros, appears in the shape of two longitudinal protuberances on either side of the mesentary along the spinal column. The protuberances consist of a series of transverse excretory tubules or nephrides. These tubules open into two pronephric ducts, or Wolffian ducts, which are running alongside the abdominal aorta. These two Wolffian or primitive male ducts open at the caudal end of the embryo into the hind-end of the alimentary canal, or the cloaca (Cut III, Fig. 2).

When the Wolffian body has almost reached its greatest development a second longitudinal duct makes its appearance by the evagination of the ventral surface of the Wolffian body. These ducts lie in close proximity of the Wolffian ducts, along the dorsal aspect of the coelom, or body-cavity, and are known as the ducts of Müller, or the primitive female ducts. The function of these canals in lowly organized animals is that of receiving from the body-cavity the ova and of evacuating them from the body. The Müllerian ducts also open into the cloaca. At the lower end the Müllerian and Wolffian ducts run in close apposition and form the genital cord. At this stage of development the embryo is thus hermaphroditic like the worms.

The metanephros, or the true kidney, appears first as an epithelial or renal evagination of the Wolffian duct on the dorsal side of the latter and near its opening into the intestinal canal or cloaca. This bud grows forward, extends headward toward the position of the Wolffian body and becomes a long, narrow tube, the ureter. The blind end branches into different tubules, each having a sacculated end. They soon assume a tortuous and convoluted form and represent the permanent kidney. The Wolffian body is now replaced in its function by the true kidney, and enters into special relations with the sexual organs, by being transformed into the genital apparatus of the male.

Before the Wolffian body has yet degenerated the mesothelial cells overlying the free surface of that body, at its upper part, and at the ventro-mesial side, assume a high columnar form and form an elongated swelling, known as the internal genital ridge. As the degeneration of the Wolffian body proceeds the genital ridge is differentiated into the indifferent sexual gland, by producing a projection upon the wall of the coelom or body cavity. This prominence is attached to the surface of the Wolffian body by a fold of the peritoneum. At this stage there is no distinction of sex. The sexual gland represents the indifferent type of the sexual apparatus.

While this metamorphosis is going on at the head-end of the sexual ducts, the caudal ends undergo also a certain degree of development.

In the early embryonic life the intestinal canal is in communication with the allantois. In fact, both form one continuous canal. When the caudal, pointed end of the intestine becomes obliterated, the allantois-duct on the ventral side, and the intestine on the dorsal side, both open into a kind of pouch, the cloaca. When the body-cavity of the embryo closes in the course of farther development, the allantois, which is the connecting link between the embryo and the placenta, enters the embryo by the small opening known as the umbilicus. Very soon the middle segment of the intra-embryonic allantois dilates and assumes the form of a spindle-shaped sac, the later urinary bladder. The portion of the intra-embryonic allantois, connecting the summit of the bladder with the umbilicus, soon becomes an impervious cord, known as the urachus. The portion of the allantois intervening between the bladder and the intestine is designated as Sinus Urogenitalis.

Into this sinus opens the short canal, connecting the lower end of the bladder with the upper end of the urogenital sinus which becomes later on the urethra. The sinus also receives the genital ducts, the Müllerian and the Wolffian ducts, and the latter’s evaginations, the ureters. Later on, owing to alterations through unequal growth, the orifices of the ureters, which originally lie in close apposition with the openings of the Wolffian ducts, change their position and are moved toward the bladder. The interval between the two pairs of ducts, the Wolffian ducts and the ureters, increases, until the ureters finally open into the bladder.

The intra-embryonic allantois has thus furnished the following organs: the solid cord urachus, or ligamentum vesico-umbilicale, the urinary bladder, the urethra and the urogenital sinus. The latter is still in communication with the intestine by means of the cloaca.

At the next step ridges or folds spring from each side of the cloaca, grow toward each other, until they finally coalesce and form a complete septum. By farther development the original epithelial septum becomes the permanent perineum. Since the intestine is now no longer in communication with the urogenital sinus, the cloaca as such disappears. At this stage the intestine ends in a blind sac. It is closed up towards the exterior by the anal membrane. Neither are urethra and genital canal in communication with the exterior. They both open into the urogenital sinus, and the latter is closed up towards the exterior by the urogenital membrane. The anal membrane soon breaks through and the rectum opens to the exterior by the way of the anus.

B. The Internal Male Sexual Organs

After the genital ridge had made its appearance, columns of cells begin to grow down into the substance of the Wolffian ridge. The columns are composed of two kinds of cells, small epithelial cells and large spherical cells, known as sex-cells. Two regions may be now recognized in the ridge, the rete-region and the sex-gland region. The cell-columns of the rete region are termed “rete-cords,” and the cell columns of the sex-gland region are called “sex-cords.” The sex-cords unite to form a complicated network and the rete-cords grow backward to the Wolffian ridge. They then develop a lumen and send off branches to the sex-cord reticulum.

The genital portion of the Wolffian body persists in the male in its entirety and serves as the efferent ducts of the testis. They open into the upper part of the Wolffian duct. The latter is retained complete. The portion nearest the testis is thrown into coils and forms a part of the epididymis, the remainder is converted into the vas deferens and the ductus ejaculatorius and the lateral outpouching of the wall, the vesicula seminalis.

The Müllerian ducts disappear completely in the male. Only the lower ends of the ducts fuse to form the sinus pocularis, or utriculus prostaticus.

C. The Internal Female Sex Organs

In the female the Wolffian body and ducts degenerate. The remainder of the body is known as the parovarium, an organ without any apparent function, while the remainder of the ducts are designated the ducts of Gartner.

The ovary is produced from the asexual stage by the following metamorphosis. The mesothelial cells on the peritoneal surface of the sexual gland change into the germinal epithelium and form the so-called egg-columns or sexual cord which represent the primitive ova.

At the caudal end, the Müllerian ducts fuse together into one, the walls, along the entire line of the union, degenerate, and the two ducts thus form a single duct, the later vagina and uterus. Until the fifth month there is no distinction between
vagina and uterus, the two organs form a single sac-like structure. At the beginning of the fifth month, a circular ridge in the wall of the sac makes its appearance and marks the division between the vagina and the uterus. When the lower portion of the two Müllerian ducts have fused to form a single canal, the utero-vaginal sac, the lumen of the vagina is still obliterated, being filled with epithelial cells. By the breaking down of the central epithelial cells, the cavity is established.

At this period a little semicircular crescentic fold attached to the dorsal margin of the aperture of the vagina arises and forms the hymen, an organ which has always played such an important rôle in the fancy of all nations.

The upper blind ends of the Müllerian ducts, with their expanded funnel-shaped mouths, diverge and form the oviducts, or the Fallopian tubes.

D. The External Genitals

At the time when the urethra, the sexual ducts and the intestine still open into the sac-like tube, the so-called cloaca, there is distinguishable on the exterior surface of the body, corresponding to the position of the cloaca, a certain depression called the cloacal depression. When the intestine is separated from the cloaca by the septum, the later perineum, the exterior cloacal depression is cut into two, the anal and the urogenital depressions. Between the urogenital depression, later called the genital groove, exteriorly and the urogenital sinus interiorly, there is only a dividing membrane, the urogenital membrane which later on breaks through and transforms the entire sinus into a shallow depression, termed the vestibule.

Before the urogenital sinus has opened to the exterior the mesenchym surrounding the urogenital depression exteriorly begins to thicken and produces an encircling elevation, the genital swelling. On the ventral side within this swelling appears a projection, the genital tubercle, which is thus surrounded by the genital swelling. The tubercle soon increases in size, so that the urogenital depression, now called the genital groove, becomes partly situated at its under aspect (Cut 5, Fig. 2). The lips of
this genital groove thicken and form the two genital folds. All these four organs are common in both sexes and represent the asexual or bisexual state of the external genital organs.

E. The Male External Genitals

In the male the genital tubercle increases enormously in size to form the penis. Its extremity becomes bulbously enlarged and forms the glans penis. The lips of the groove or rather of the vestibule, since by this time the urogenital membrane had broken through and had transformed the sinus urogenitalis into the vestibule, the so-called genital folds, meet together and fuse, thus converting the vestibule and the groove into the terminal portion of the male urethra and bringing it about, that the ductus ejaculatorii and the sinus pocularis open upon the floor of that passage. The prostate, consisting of several independent glands, has also its openings at this point. In its development the prostate belongs to the urethra as well as to the sinus urogenitalis. The two genital swellings are brought closer together in the male and form the scrotum, a sac containing two separate pouches into which the testes descend.

F. The Female External Genitals

In the female the vestibule, or the shallow depression which was formed through the breaking through of the urogenital membrane, remains open throughout life, and is termed the vestibule of the vulva. From the sides of the lower part of the sinus a pair of evaginations are formed and give rise to the Bartholinian glands. The vestibule being in fact the open sinus urogenitalis, the urethra and the vagina naturally have their orifices in the same.

The genital tubercle ceases to grow in the female and becomes the clitoris. The genital folds or the lips of the vestibule become prolonged and form the labia minora or the nymphae. The genital swelling increases in size through adipose and fibrous tissue. The part situated on the ventral side of the clitoris becomes the mons veneris, while the lateral parts are converted into the labia majora of the vulva.