COPULATORY ORGANS OF THE AMNIOTA. 
27 
the penis, which, moreover, as indicated in fig. 10, is continuous with the ventrilateral 
wall of the proctodseum. The preputial room of the Marsupials is therefore, strictly 
speaking, not completely homologous with the similar room of the Monotremes. 
Vulva and penis of the Marsupials, and of the Placentalia mentioned above, are still 
surrounded by the same external fold of skin and by the same sphincter of the anus. 
The shallow vestibulum is the last remnant of a cloaca. 
In most Placentalia the cloaca is abolished by the development of a true peringeum 
which reaches the outer surface and secures a complete separation of the anal and 
urino-genital openings. The vestibulum is broken up; its dorsal portion forms the 
anus, whilst its ventral half, owing to the close approach of the urino-genital sinus 
to the surface, becomes the shallow “ vestibulum ” in the female, and in the male it is 
partly recognisable as the preputial room. The labia minora with the frenulum 
clitoridis are a remnant and modification of the sauropidan fold F. Lastly, in the 
females of these Placentalia, in which, like in Myogale, Talpa, Galeopithecus, and 
certain Lemurs, the clitoris is perforated by the urethra, the sinus urino-genitalis 
itself is divided into a dorsal or genital and a ventral or urethral half, both openings, 
however, retaining their intravestibular position. 
A summary of the anatomical and physiological differences presented by the cloacal 
region of the various Amniota is given in the following tabic :— 
Usually the cloaca is defined as a chamber at the terminal portion of the rectum, 
into which open the rectum, the ureters, and the genital tubes. 
Balfour remarks that “ in all Vertebrata the cloacal section of the alimentary 
tract, which receives the urino-genital ducts, is placed in communication with the 
exterior by means of an epiblastic invagination constituting the proctodcBum (vesti¬ 
bulum cloacse, s. anal chamber). The original boundary between the epiblast of the 
proctodseum and the hypoblast of the judmitive cloaca becomes obliterated after the 
two have become placed in free communication. The hypoblastic section of the cloaca 
of birds, which receives the openings of the urino-genital ducts (our chamber U.D.), 
is permanently marked oft 1 by a fold from the epiblastic section or true proctodceam, 
with which the bursa Fabricii communicates.” This fold is the one described in this 
essay as F. It occurs with modifications in all Sauropida, and even in the Mammalia. 
The fold rc separates the primitive cloaca from the rectum. Considering, first, that 
in the Crocodilia the genital tubes open decidedly into the proctodaeal portion ; 
secondly, the configuration of the Chelonian cloaca ; thirdly, the occasional use as 
urinary receptacle of the bursa Fabricii ; lastly, the condition prevailing in the 
Monotremes, we have to conclude that the vestibulum forms part of the cloaca. 
The whole cloaca consequently consists of three successive chambers, which may be 
distinguished as follows :— 
