THE URO-GENITAL SYSTEM 379 



epithelium, become reinforced from behind by the prolifera- 

 tion of connective tissue, containing nerves and blood-vessels ; 

 and thus are formed mounds projecting into the ccelom, cov- 

 ered with germ cells. This association becomes more intimate 

 through the intrusion of the germinal epithelium into the in- 

 terior in many places, where the cells receive the nourishment 

 necessary for their complete development. 



The germ cells themselves are of two sorts, ova and sper- 

 matozoa, and their differences in form and size necessitate a 

 more or less apparent difference in the organs that produce 

 them, the ovaries and testes respectively. In certain cases 

 among cyclostomes the same germ gland produces both sorts 

 of germ cells, although at different times, but with this ex- 

 ception the sexes are normally separate. The occasional oc- 

 currence of a few cells of one sort in a gland which normally 

 produces the other, as the development of a few ova on the 

 side of a testes, or vice versa, occurs as an anomaly among 

 many of the lower vertebrates, and this phenomenon, taken 

 in connection with the cases among the cyclostomes just cited, 

 has led to the possible theory that the ancestors of vertebrates 

 were hermaphroditic, as is the case in many invertebrates, 

 but there is little else to indicate this. Reported hermaphro- 

 dites among higher vertebrates are usually if not always ap- 

 parent rather than real and are in fact malformations due to 

 some error in development affecting mainly the external parts. 



All that is essential for the production of a new organism is 

 the complete and intimate union of the two germ cells, one 

 of each sort, but the varied environment of the parents often 

 makes it a problem to arrange the means by which this may 

 be accomplished. It offers the least difficulty in the case of 

 aquatic forms, for all that is here necessary is to liberate the 

 cells of both sorts into the water, in which the union can be 

 easily effected, since the water furnishes the fluid medium nec- 

 essary for the locomotion of the spermatozoa. Often, too, 

 such animals associate in pairs and develop elaborate instincts 

 which insure the discharge of the two products in close prox- 

 imity to one another. 



