194 THE STRUCTUKE OF MAN 



extent united, the union being first effected at a middle point, 

 before the ducts themselves open into the urinogenital sinus. 

 [Those portions of the oviducts situated above this point of union 

 become converted into the uteri and Fallopian tubes, and those 

 below into the vaginae.] 



Among the Marsupials there arises at this point a median 

 vaginal sac, and neither the upper (uterine) nor the lower (vaginal) 

 portions unite further. For this reason these animals are fre- 

 quently classified as the Didelphia. In all the higher and truly 

 placental Mammals the union extends backwards to form a 

 single median vagina [as is expressed in the application to them 

 of the term Monodelphia]. It also extends forwards, giving rise 

 to a single median uterus as we ascend in the series (cf. Fig. 

 104). Man and the Primates are among those monodelphous 

 Mammals in which the two uteri as a rule completely unite : 

 but abnormal forms of uterus, known as uterus duplex, bilocularis, 

 subseptus, bipartitus, incudiformis, arcuatus, and bicornis, not 

 infrequently occur, at any rate in Man. These are but the 

 expression of arrested development, arrested, that is, at stages 

 corresponding with those of the gradual fusion of the originally 

 separate Miillerian ducts effected during the course of long 

 geological periods. The uterus simplex is the normal condition 

 in the Primates of the present time. 



In the uterus simplex, traces of the primitive paired condition 

 of the Miillerian duct are still found in the paired Fallopian 

 tubes (pd., Fig. 104, C), and in the longitudinal ridges of the 

 cervix uteri and of the vagina (columnee rugarum). 



HYMEN 



The primitive significance of the fold of mucous membrane 

 termed the hymen, which lies within the entrance of the vagina 

 in the female and more or less completely closes it, is by no means 

 clear. The only thing that can be stated with certainty is that 

 it is coincident in disposition with the elevation of the urethral 

 mucous membrane of the male known as the colliculus seminalis. 



[It is an interesting fact that a similar and complete fold is present within 

 the base of the oviduct in the virgin state of the lower Fishes (Sharks and 

 Rays).] 



THE CLOACA 



At a certain stage in the development of Man the urinogenital 

 ducts and intestine open posteriorly into a common chamber, the 





