THE DETERMINATION OF SEX 293 



phores, flat-worms, round-worms, annelids, molluscs 

 and crustaceans. It is, however, the exception rather 

 than the rule and must be viewed as a modification of 

 the bisexual condition necessitated to insure insemina- 

 tion in animals poorly adapted to bring about typical 

 fertilization of the eggs. 



Sometimes hermaphrodites are female in appearance 

 and again they resemble more closely the males of the 

 group to which they belong. In certain Nematodes, 

 for example, Rhabdites aberrant an occasional male is 

 found among thousands of hermaphrodites of female 

 appearance. In this worm Miss Krueger has shown 

 that occasionally there is a failure of one chromosome 

 to become incorporated in one of the second sper- 

 matocytes. Spermatozoa resulting from such deficient 

 spermatocytes may be the cause of these occasional 

 male zygotes. Since our knowledge of the chromosomes 

 in hermaphroditism is deficient, it is hardly worth 

 while at present to speculate on the mechanism which 

 produces such individuals. 



That the sexual tendencies of hermaphroditic forms 

 are often in a sensitive balance, influenced by external 

 conditions, is shown by the experiments of Baltzer on 

 Bonellia and by Gould on Crepidula. 



In the marine worm Bonellia there are produced 

 minute motile larvae with hermaphroditic possibilities. 

 If these free-swimming larvae find the proboscis of a 

 female Bonellia they attach themselves thereto and 

 develop into minute males after a parasitic existence of 

 about four days. If, however, no proboscis is encoun- 

 tered, the motile larva sinks to the bottom and develops 



