32 
DR. H. GADOW ON THE CLOACA AND ON THE 
separating uro-proctodseal fold F. The epiblastic origin of the proctodseum explains 
the frequent presence on the copulatory organs of epiblastic products, like horny 
spines, scales, sebaceous glands, and its highly sensitive nature. 
That there existed a stage of phylogenetic development, during which the lateral 
portions of the protrusible tube became stronger, and that they specialised themselves 
into a right and into a left imperfect intromittent organ, the walls of which then— 
being stowed away in recessus of the proctodseum—escaped being used also as the 
walls of the efferent fsecal chamber. 
That subsequently in one group, viz., in Snakes and Lizards, these penes were 
shifted back towards the tail and were developed into separate organs. After this 
had taken place they could not well meet and fuse with each other in the posterior 
dorsal corner of the anus, since then them bases would be too far removed from the 
openings of the vasa deferentia, with which they are now still connected by longi¬ 
tudinal folds forming the continuation of their semi-canals. 
That in the other groups of Amniota the two primitive lateral erectile flaps 
approached each other towards the ventral side, and thus arrived at the anterior or 
ventral side of the cloaca. Their fusion, beginning probably at the basal part, which 
at the same time was further withdrawn from the surface, secured the reception of 
the sperma from both vasa deferentia into one canal. 
That this ventral copulatory organ, now in the Reptilia restricted to the Crocodilia 
and Chelonia, has been inherited by the Avian stock, and has been specialised in 
the various ways fully and correctly described by Joh. Mueller. The struthious 
form comes nearest to that of the monimostylic Reptiles, whilst that of the other 
Ratitae and of the Lamellirostres shows great specialisation in being evertible. The 
comparison of the organ, as it is found in various other Carinatse, e.g., in the 
Tinamoos, the Cracidae, in Phoenicopterus, Platalea, Ciconia, shows a gradual diminu¬ 
tion in size and a simpler structure, with all the appearance of a degraded organ. 
Lastly, in the majority of Birds, especially in the highest, it has disappeared, and the 
primitive way of everting the cloaca is again resorted to. The degeneration and final 
loss of such an organ, the development of which must have been caused and favoured 
equally by natural and by sexual selection, is a fact which we would not have arrived 
at by d priori considerations. 
That the presence of a clitoris is not due to an originally hermaphroditic condition, 
but to direct paternal inheritance, and that it is preserved because of its hedonic 
nature. 
That the extraordinary resemblance of the copulatory organs and the various 
cloacal chambers of the Monotremata to those of the Chelonia and young Crocodilia 
can hardly be explained by homoplastic coincidence, but that it strongly urges the 
phylogenetic relationship of the Mammals with the Reptiles. Tins, however, is only 
one more link in the long chain which, being anchored in the triassic Theriomorpha, 
in spite of Mono- and Amphicondylism, makes the Amniota more akin to each other 
than to the Amphibia. 
