194 THE STRUCTURE 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 vaginee. | 
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 (od., Fig. 104, C), and in the longitudinal ridges of the 
cervix uteri and of the vagina (columne 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 
