346 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



blood, the whole structure, the distal end of which is free, can be 

 protruded from the cloaca and retracted by suitable muscles (fig. 344) . 

 In the monotremes (fig. 345, /) the penis is still cloacal in position 

 and the urogenital sinus still communicates with the cloacal cavity. 

 But the advance' is made that the groove of the sauropsida has been 

 converted into a tube which carries the urine as well as the sperm. 

 The whole structure can be protruded and retracted again into a 

 sheath formed from the loose mucous membrane of the cloaca. In 

 the other mammals the connection of the urogenital ducts with the 

 alimentary tract is lost and the cloaca disappears. In the lower 

 mammals (figs. 341, 345, //) the retractile condition is retained but 

 in the higher the organ is permanently external (fig. 345, ///). In 

 the marsupials the tip of the penis is frequently bifurcate, corresponding 

 to the two vaginae of the female. In many rodents (fig. 341, op), 

 bats, many carnivores, whales and some of the primates a penis bone 

 is developed in the middle line of the intromittent organ. 



HERMAPHRODITISM. 



Individuals of either sex which have assumed some of the external 

 or secondary sexual characters of the other sex are sometimes spoken 

 of as hermaphrodites, especially in the case of mammals if the copu- 

 latory organs be concerned. This is not true hermaphroditism, which 

 consists in having both ovarian and testicular organs or tissues in the 

 same individual and as a consequence the ability to produce both eggs 

 and spermatozoa. There may be both kinds of tissue in the different 

 parts of the same gonad, or the two may be intermingled (ovotestis) 

 or the gonads of the two sides of the body may be of different sexes. 

 Both ovaries and testes may be functional at the same time, or one 

 may be functional at one time and the other at another (proterandric 

 hermaphroditism) . 



There is an enormous literature dealing with the problem of the 

 determination of sex. Almost every conceivable possibility has been 

 invoked to account for fact that one individual is male and another 

 female chance, multiple impregnation, difference in age of parents 

 or of eggs and spermatozoon, matters of temperature and nutrition, 

 etc. Within the last few years there has been a strong tendency to 

 regard the matter as determined at the time of impregnation of the 

 egg and to depend upon differences in chromosomes. 



