340 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



double nature is shown only by the separate openings of the two 

 Fallopian tubes. 



In the monotremes the primitive relation of urogenital sinus and 

 rectum both emptying into the cloaca (figs. 338, 339, A) persists 

 through life, the result being a single external opening for the digestive 



FIG. 339. Uteri of (A} Ornithorhynchus; (B) Halmaturus; (C) sheep and (Z>) Inuus, 

 after Gegenbaur. b, bladder; bo, bursa ovarica; c, cornua uteri; d, cloaca; I, ligament of 

 ovary; o, ovary; od, oviduct (Fallopian tube); pv, processus vaginalis; sus, sug, urogenital 

 sinus; u, uterus; ur, ureter; v, vagina; vc, vaginal canals. 



tract and the urogenital ducts, whence the name monotreme. In 

 all other mammals the cloaca becomes divided by a partition, the 

 perinasum, between the urogenital and the rectal portions, there thus 

 being formed two external openings. However, in certain mammals, 

 as in marsupials and some rodents, both may be enclosed in a common 



