THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 111 
gans arise by differentiation, fusion, and atrophy: thus, the 
cloaca or common cavity of the genito-urinary ducts is divided 
by a septum (the perineum externally) into a genito-urinary 
and an intestinal (anal) part; the penis in the male and the 
corresponding clitoris in the female appear in the region of the 
cloaca, as outgrowths which are followed by extension of folds 
of integument that become the scrotum in the one sex and the 
labia in the other. 
The urethra arises as a groove in the under surface of the 
penis, which becomes a canal. The original opening of the 
urethra was at the base of the penis. 
M, 
Fia. 132, Fia. 133. 
Fies. 130 to 183.—Diagrams illustrating the evolution of the posterior passages (after Landois 
and Stirling). 
Fie. 130.—Allantois continuous with rectum. 
Fig. 131.—Cloaca formed. 
Fic. 182,—Early condition in male, before the closure of the folds of the groove on the poste- 
rior side of the penis. 
Fie. 133,—Early female condition. P 
A, commencement of proctodeum; ALL, allantois; B, bladder; C, penis; CL, cloaca; 
M, Millerian duct; R, rectum; U, urethra; S, vestibule; SU, urogenital sinus; V, vas 
deferens in Fig. 132, vagina in Fig. 133. 
In certain cases development of these parts is arrested at 
various stages, from which result abnormalities frequently re- 
quiring interference by the surgeon. 
The accounts of the previous chapters do not complete the 
history of development. Certain of the remaining subjects 
that are of special interest, from a physiological point of view, 
will be referred to again; and in the mean time we shall 
consider rather briefly some of the physiological problems of 
this subject to which scant reference has as yet been made. 
Though the physiology of reproduction is introduced here, so 
that ties of natural connection may not be severed, it may 
very well be omitted by the student who is dealing with embry- 
