26 
DR. H. GADOW ON THE CLOACA AND ON THE 
modifications towards a higher type, in the Monotremata (cf. fig. 27). The differences 
lie chiefly in the relative size of the chambers UD and PD, and in the circumstance 
that the penis is lodged in a special pouch, which communicates with the rest of the 
vestibulum through a narrow opening close to the ventral brim of the anus. We can 
derive the wall PF, which separates the penis pouch from the rest of the whole vesti¬ 
bulum or proctodasum, from the ventral half of the fold F of the Sauropida.; the only 
difference is that this fold, which in Crocodiles, Birds (cf. figs. 14, 15, and 10), and 
Chelonians, gradually passes over into the loose coating of the penis, and thus forms 
the beginning of a preputial sheath, has in the Monotremata become considerably 
elongated in a longitudinal instead of a transverse direction. This I have tried to 
explain diagrammatically by figs. 17 to 30. Such a reduplication of the loose penial 
coating would almost completely separate the copulatory organ from the urodseum, 
and, in fact, these folds leave in the Monotremata a small opening near the root of the 
penis groove for the reception of the sperma from the sinus urogenitalis, but not for 
the urine and for the eggs. The walls of the urodseum, into which the urogenital 
sinus opens, are very thin, and the muscular, chiefly longitudinal, coating is likewise 
weak, with the exception of the voluntary striped muscle on its ventral aspect 
(fig. 28). This chamber UD, the urodseum, receives in the middle of its ventral wall 
the urine and the eggs, but not the sperma. Towards the vestibulum PD it is closed 
by a circular constriction and partly developed fold F, and the terminus of the rectum 
is marked by a strong circular muscle, which forms a powerful innermost sphincter rc. 
Whilst in Echidna the rectum shows a large dilatation above the fold rc, and forms 
there a true rectal chamber or coprodseum for the retention of the faeces, the latter 
in Ornithorhynchus probably mix with the urine in the chamber UD, which in this 
genus is very capacious in opposition to the only slightly dilated rectum. 
Bridge’s remark that the existence of a complete partition between the rectum 
and the urino-genital opening is a characteristic point of all Mammalia, including those 
which possess a cloaca, is not correct, because the urino-genital sinus and the rectum 
are separated from each other just as much or as little in the Monotremata as in the 
Chelonia. Not much progress towards a higher type show the Marsupialia (fig. 28), 
and even some Undents, Insectivores, and Lemurs. The chamber UD becomes con¬ 
siderably shortened, and at the same time the walls in the corner between the rectum 
and the sinus urogenitalis, represented in the Tortoise by the fold p, fig. 25, transform 
themselves into a growth, which, progressing more and more towards the anus, results 
in the almost complete division of the former cloaca into a dorsal or faecal and into a 
ventral or exclusively urogenital chamber. The beginning of a perinseum is conse¬ 
quently derived from the fold p, i.e., a fold inwards, above, or headwards from the 
opening of the urogenital sinus, and is not to be confounded with the partition PF of the 
Monotremata in fig. 27 ; although, of course, we have to bear in mind that the fold p 
in the Tortoises, as shown in fig. 11 at p, goes over into the lateral loose coating of 
