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, I) 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, IJ) the retractile condition is retained but 
in the higher the organ is permanently external (fig. 345, JI). In 
the marsupials the tip of the penis is frequently bifurcate, corresponding 
to the two vaginz of the female. In many rodents (fig. 341, of), 
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 cepu- 
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. 
