398 HISTORY OF THE HUMAN BODY 



tion and thus form the prelude to the development of a gen- 

 uine copulation. 



It is evident, however, that with the complete relinquish- 

 ment of an aquatic life, and the subsequent impossibility of 

 employing an external vehicle for the conveyance of the sper- 

 matozoa, some method must be found by means of which the 

 seminal fluid may be conveyed direct from the male to the 

 female; and this process, beginning with the most natural 

 stage of the approximation of the two unmodified cloacae, 

 would develop first a temporary evagination of a portion of the 

 inner cloacal wall, and then a permanent modification of this 

 evaginating portion; a development which would naturally 

 take place in the male alone, as the producer of the fluid to be 

 transferred. There thus arises for the first time in vertebrates 

 an intromittent organ or penis, three distinct types of which 

 are found; these appear to have arisen independently, al- 

 though in all cases by a modification of the cloacal wall. The 

 first is seen in. those highly specialized burrowing amphibians, 

 the Gymnophiona, and consists of a protrusible tube worked 

 by muscles ; the second is that of lizards and snakes, and is in 

 the form of two lateral protrusible sacs, the walls of which are 

 often cornified, and possess a spiral groove for the convey- 

 ance of the spermatic fluid ; the third occurs in its simplest 

 form in turtles and crocodiles and suggests a terrestrial origin 

 for both groups. This latter is the type from which the penis 

 of both birds and mammals is derived, and may be described 

 more at length. Owing to the imperfectly understood law of 

 sexual homology which obtains among vertebrates, this organ, 

 sometimes termed the phallus to distinguish it from the other 

 types, exists also in the female in a much reduced form, and 

 is termed the clitoris. Although useless as an intromittent 

 organ, it reflects the peculiarities of the male organ and in the 

 various groups often shows in a reduced form the characteris- 

 tics developed by the latter. 



The phallus develops from the ventral wall of the cloaca 

 and consists of a longitudinal thickening of fibrous tissue, the 

 corpus fibrosum, upon which rests a mass of cavernous (erec- 



