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.
CUT IV.