382 COMPARATIVE ANATOMY 



fibrous body, bifurcated at the base. In Dromaeus and Rhea there 

 is an aperture at the apex of the penis leading into an elongated 

 and curved blind sac, in which is a furrow, lined by cavernous 

 tissue, continuous with the groove on the dorsal side of the 

 organ. In the D.uck and Swan the spiral penis is essentially 

 similar to that of Dromseus and Rhea. The absence of the blind 

 sac in the Ostrich, however, is probably a secondary modification. 

 A clitoris is present in the female of the above-mentioned Birds. 



The penis of Monotremes may be best understood by imagin- 

 ing a hypothetical form intermediate between it and that of Croco- 

 diles and Chelonians (Fig. 305, B). We must suppose that 

 a sac-like outgrowth into which the ureter and vasa defe- 

 rentia open has become developed from the ventral cloacal wall at 

 the base (anterior end) of the penis, the groove in which has 

 become converted into a canal. The Monotreme condition is 

 reached by the sac elongating to form a urinogenital canal, into 

 the distal end of which the urinary and genital ducts and the 

 bladder open (c, D). The penis consists of an unpaired . fibrous 

 body enclosing the canal, and is only loosely surrounded by the 

 mucous membrane of the cloaca, so that it can be protruded from 

 and retracted into a sheath in which the apex or glans lies. 



In Echidna, cavernous tissue is present in the glans ; and in Ornitho- 

 rhynchus the latter is bifurcated and covered with soft spines, the seminal 

 canal opening in a groove on each fork by numerous fine canals situated on 

 papillae. 



A clitoris is present in the female of all Mammals. 



In Marsupials (Fig. 306, A), the penis-sheath opens directly 

 on to the surface of the body ; the opening of the urinogenital 

 canal into the cloaca has become closed, and is continuous with 

 the seminal tube or urethra of the penis. The fibrous body 

 is paired, and both it and the walls of the penial urethra are com- 

 posed of cavernous tissue. 



Amongst Flacental Mammals the penis of Rodents (Fig. 

 306, B) and Insectivores comes nearest to that of Marsupials. The 

 paired fibrous body (corpus cavernosum) bifurcates proximal ly 

 to form two crura, which are nearly always attached to the 

 ischia. The opening of the penis-sheath gradually becomes further 

 separated from the anus, and is situated more on the ventral side of 

 the body (compare B and c), the penis itself lying horizontally 

 along the abdomen. In Primates the organ becomes more or less 

 free from the body-wall, and either its distal end (Apes, c), or the 

 whole of it (Man, D) hangs freely, and the sheath forms a double 

 tube-like investment, the foreskin or prepuce, over the glans. 



In the course of development, the penis of Marsupials and 

 Placental Mammals passes through stages which resemble succes- 

 sively those which are permanent in Crocodiles and Chelonians 

 and in Monotremes. It arises from a " genital prominence" on the 



