THE URO-GENITAL SYSTEM 



389 



association with the openings of the ureters. There are no 

 vaginae, although the urogenital sinus serves functionally 

 as one. In the oviducts the small oval eggs, 3.5 x 4.0 mm. in 

 diameter when they leave the ovary, are retained for some 

 time and increase in size to about 12x15 mm. through the 

 absorption of nutrient fluids secreted by the oviducal walls. 



The next stage is that of the marsupials, in which the two 

 ducts are still distinct, but each shows a differentiation into 

 oviduct, uterus and vagina. The two vaginal orifices open 



DUPLEX 



BIPARTITUS 



SIMPLEX 



FIG. no. Different types of mammalian uterus: explanation in text. 



into a common urogenital sinus, which is here prolonged into 

 a canal, and opens to the exterior separately from the rectum. 

 Here for the first time the complete separation of these two 

 canals, urogenital and alimentary, is met with, since in mono- 

 tremes, although internally distinct, there is externally a com- 

 mon cloacal orifice. The partition separating the two becomes 

 thicker and of more importance in the higher mammals, and 

 forms the perin&um. 

 Above the marsupials the two ducts fuse into one at their 



