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 
Fic. 339.—Uteri of (A) Ornithorhynchus; (B) Halmaturus; (C) sheep and (D) Inuus, 
after Gegenbaur. 8, bladder; bo, bursa ovarica; c, cornua uteri; cl, cloaca; /, ligament of 
ovary; 0, ovary; od, oviduct (Fallopian tube); pv, processus vaginalis; sus, sug, urogenital 
sinus; “, 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 
perineum, 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 
