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_