THE URINARY BLADDER. 345 



band of the urinary vesicle (ligamentum vesico-umbilicale 

 medium}, a rudimentary organ which extends as a solid 

 cord from the top of the urinary bladder to the navel. The 

 lower part of the allantois-pedicle (the " urachus ") remains 

 hollow, and forms the urinary bladder. At first, in Man, 

 as in the lower Vertebrates, this organ discharges into the 

 last section of the posterior intestine, and there is, there- 

 fore, a true " cloaca," receiving both urine and excrement ; 

 but, among the Mammals, this cloaca is permanent only in 

 the Cloacal Animals, or Monotremes, as in Birds, Reptiles, 

 and Amphibia. In all other Mammals (Marsupialia and 

 Placentalia) a transverse partition forms at a later period, 

 and separates the urinary-sexual aperture in front from the 

 anal aperture behind. (Cf. Chapter XXV.) 



