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.
CUT III.
Fig. 1, horizontal cut through embryo; Fig. 2, vertical cut; schema of indifferent stage.
Wb, Wolffian body; Wd, Wolffian duct; Md, Müllerian duct; gg, genital gland; m, mesentery; i, intestine; sc, spinal cord; ch, chorda; ao, aorta; a, allantois; clo, cloaca; k, kidney.
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.