THE BLADDER 



205 



Thf hhiddcr occurriii^^ ain()ii<i; the aiiiiiiote.s has l)(.'en called an 

 allantoic bladder, hut enibryological studies have shown that the 

 allantois is not really concerned with the formation of the bladder of 

 these forms. Ophidia, Crocodilia, many Lacertilia, and A\es have 

 no bladder. Turtles, some lizards, and all mammals have one. 

 The bladder of the monotremes is said to be homologous with that 

 of amphibians. 



Mucosa 



^XSitbinucosa 





lO -^^ kgO' 0»j?; cD^^-«^ 



X Longitudinal 



5 



P Circular 



^ Longifudinal 



Adventitia 



Fig. 1.30. — Diagram of mammalian bladder. 



An idea of the development of the mammalian bladder may be 

 gained from a study of its develo])ment in man. In embryos, the 

 posterior end of the embryonic gut is somewhat expanded and \-en- 

 trally lies against the ectodermal body wall. Later in this region 

 an endodermal diverticulum from the cloaca will meet an ectodermal 

 invagination to form the anal opening. From the bend where each 

 mesonephric duct joins the cloaca a burl will form, and from this will 

 develop the ureter of the metanephric kidneys already described. 

 Somewhat later the cloaca in this region subdivides into two pas- 

 sages entirely separated from each other. The more dorsal of the 

 two forms the rectum, and the \entral forms a urogenital sinus. 

 This occurs when the embryo is about eight weeks old in himians, 



