THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 435 



represents a secondary constriction after the ureter is formed since both evag- 

 inations are hollow from the beginning (p. 394), but the cause of the constric- 

 tion is not understood. The atresia results in dilatation of the portion of the 

 ureter on the side toward the kidney. 



Abnormal situations of the openings are sometimes seen, the explanation 

 of which is to be found in the relations of these tubes to the mesonephric ducts, 

 to the cloaca, and to the Miillerian ducts. In the male the ureters may open into 

 the seminal vesicles, the prostatic urethra, or the rectum. If one recalls that 

 the ureter arises as an evagination from the mesonephric duct near the opening 

 of the latter into the cloaca (p. 394), that the cloaca becomes separated into a 

 dorsal part (the rectum) and a ventral part (the urogenital sinus) (p. 403), and 

 that the proximal end of the mesonephric duct is so far taken up into the wall 

 of the urogenital sinus (or bladder) that the ureter opens separately (p. 403) , it is 

 readily seen that any interference with these normal processes of development 

 will result in abnormal opening of the ureter. If the ureter does not become 

 separated from the mesonephric duct, it will open into the deferent duct, (vas 

 deferens), the latter being the proximal part of the mesonephric duct. And 

 since the seminal vesicle is an outgrowth from the proximal end of the meso- 

 nephric duct, the opening of the ureter is likely to be associated with the vesicle. 

 If the separation between the ureter and mesonephric duct is complete, but 

 the opening of the ureter does not migrate cranially on the wall of the bladder, 

 the opening comes to lie in the wall of the prostatic urethra. If the wall 

 (urorectal fold) separating the urogenital sinus and rectum is situated too far 

 dorsally, the opening of the ureter comes to be in the wall of the rectum. (Con- 

 sult Figs. 360, 361, 362, 363.) 



In the female the ureters may open into the urethra, the vagina, or the uterus. 

 The explanation of the opening into the urethra is the same as in the male 

 (see preceding paragraph) . The opening into the genital tract is probably to 

 be explained on the ground that the ureters fail to migrate cranially along 

 the wall of the urogenital sinus to the bladder, and as the fused ends of the 

 Miillerian ducts enlarge to form the uterus and vagina, the openings of the 

 ureters are taken up into their walls. 



The Bladder. — Absence of the bladder is very rare. Abnormal small- 

 ness, due to imperfect dilatation of the urogenital sinus (p. 404), is not infre- 

 quent. 



The urachus, which represents the portion of the allantoic duct between 

 the bladder and the umbilicus (p. 404), not infrequently persists as a whole or 

 in part, giving rise to certain anomalous conditions in the region of the middle 

 umbilical ligament. The urachus may persist as a complete tube, lined 

 with epithelium, thus forming a means by which urine can escape at the 

 umbilicus. This condition is usually associated with obstruction of the 



