X THE EXCRETORY SYSTEM 209 



the aorta of a frog in which the renal arteries were tied. 

 This observer, in repeating Nussbaun's experiments, failed to 

 confirm some of the latter's results and considered that the 

 conclusions that were founded upon them were not estab- 

 lished on a firm basis. The subject is one that demands 

 renewed investigation. 



The Bladder. - - The bladder is a thin-walled, bilobed sac 

 attached to the ventral side of the cloaca, just below the 



FlG. 57. Diagram of the bladder and rectum of the frog; A, from the 

 side ; B, from below ; Bl, bladder ; C, cloaca ; K, rectum ; S, sphincter 

 muscle ; U, ureter ; Ut, uterus. (Modified from Gaupp.) 



openings of the ureters. It arises as an outpushing of the 

 ventral wall of the cloaca like the allantois of the embryos 

 of the higher vertebrates, with which it is regarded as homolo- 

 gous. It is surrounded by peritoneum which is continued 

 as a median dorsal sheet attaching it to the rectum ; a ven- 

 tral sheet of peritoneum connects it with the ventral body 

 wall, and a lateral peritoneal extension on either side joins 

 the sides of the bladder to the dorso-lateral regions of the 

 body wall. 



The inner surface of the bladder is lined with a layer of 

 epithelium about three cells thick (List), the inner layer 



