20 6 NORMAL HISTOLOGY. 



tract as far as, but not including, the bladder being, therefore, of 

 mesodermic origin. 



The development of the urinary bladder is connected with the 

 history of the allantois. The latter grows out of the hind gut as a 

 diverticulum which reaches conspicuous dimensions, in many embryos 

 appearing as a large, flask-shaped sac ; in man, however, the allan- 

 tois is never free, but grows as a stalk in close relations with the other 

 structures passing through the umbilical opening. The portion of 

 the allantoic canal lying within the embryo becomes differentiated 

 into three divisions : the much larger middle segment greatly 

 dilates and essentially constitutes the bladder ; the outer division, 

 extending from the bladder to the umbilicus, forms the atrophic 

 urachus, while the narrow inner portion establishes communi- 

 cation between the bladder and the uro-genital sinus and assists 

 the latter in forming the urethra proper. The primitive ureter, 

 which at first opens in company with the Wolffian duct into the uro- 

 genital sinus, changes the position of its exit until the tube finally 

 assumes its permanent relations and opens into the bladder. The 

 epithelium lining the allantois is a direct extension of the entoderm 

 of the primary gut ; the allantoic derivatives, including the bladder 

 and the urethra, therefore, are clothed with entodermic cells; 

 the muscular and connective tissues of their walls, however, are con- 

 tributions from the mesoderm. 



The short female urethra, extending from the bladder to the 

 upper part of the vestibule, the representative of the uro-genital 

 sinus, corresponds with the primary vesical canal, and is the ure- 

 thra proper. In the male subject this passage is supplemented 

 and greatly lengthened by the approximation and closure of the folds 

 by which the sinus is converted into the narrow canal extending to 

 the end of the penis. The male urethra, therefore, consists of two 

 morphologically distinct divisions : the urethra proper, which in- 

 cludes that portion of the adult canal lying between the neck of 

 the bladder and the uterus masculinus or sinus pocularis, this division 

 being the strict homologue of the female urethra ; and the remain- 

 ing part of the canal, or supplementary urethra, which represents 

 the closed and extended uro-genital sinus. 





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