THE URO-GENITAL SYSTEM 399 



, 

 tile) tissue in the form of two lateral ridges, the corpora 



cavernosa, with a median groove between them, the seminal 

 groove. This entire organ is somewhat tongue-shaped and 

 free at the tip, and is capable of considerable protrusion be- 

 yond the cloacal orifice. The urogenital sinus, bearing the 

 openings of the ductus deferentes, opens into the seminal 

 groove near its proximal end. 



Although the phallus of these reptilian forms seems at first 

 sight quite distinct from that of mammals, and although there 

 exist at present no transition forms among adult animals, 

 the development of these parts in mammals supplies the missing 

 portions of the history and substantiates the homology. The 

 essential change is that of the conversion of the spermatic 

 groove into a complete tube, which is accomplished by the 

 increase in size of the lateral ridges and their subsequent 

 fusion, a process repeated during early development. The 

 failure to complete this produces the condition known as 

 hypospadias, and is thus seen to be a case of arrested develop- 

 ment, the retention of the reptilian stage. 



The relative position of the penis changes completely during 

 its mammalian history from a post-scrotal one with the free 

 end directed posteriorly to one that is pre-scrotal and directed 

 anteriorly. The first of these positions is similar to that of the 

 turtles and crocodiles and is seen in the monotremes, and to 

 a lesser extent in marsupials ; the latter position is characteris- 

 tic of placental mammals. This change may be made clear 

 by the accompanying diagrams (Fig. 112). 



In the monotremes the conditions are still essentially rep- 

 tilian. There is a common cloaca and the penis projects a 

 little from its ventral wall. The ureters, ductus deferentes and 

 urinary bladder form a common duct which under normal con- 

 ditions serves merely as a passage for the urine. This duct 

 is morphologically the urethra as far as the entrance of the 

 ductus deferentes and ureters ; beyond this point it is morpho- 

 logically the urogenital sinus. The erection of the penis, 

 through the slight lengthening of its inner end, closes the 

 entrance into the cloaca, but continues the urogenital canal 



