Basis of Sex Determination 215 



to influence the formation of sex characters and of the 

 sex glands themselves. 



The most striking observations in this direction 

 were made by Baltzer on a marine worm, Bonellia. In 

 this animal the two sexes are very different, the male 

 being a tiny parasite, a few millimetres in length, which 

 spends its life in the uterus of the female, whose size is 

 about five centimetres. A female carries as a rule 

 several and often a large number of the male parasites 

 in its uterus, which indicates that the males prevail 

 numerically. The fertilized eggs of the animals are 

 laid in the sea water where the larvae hatch. At the 

 time of hatching all larvae are alike. The differentiation 

 of the larvae into the dwarf males and the giant females 

 can be determined at will. The larvas have a tendency 

 to attach themselves to the proboscis of the female as 

 soon as they hatch. If given a chance to do so and if 

 they stick to the proboscis for more than three days they 

 will develop into males, which soon afterwards creep 

 into the female where they continue their parasitic 

 existence. If, however, no adult female Bonellia is put 

 into the aquarium in which the larvse hatch, about 

 ninety per cent, of the larvae will, after a period of rest, 

 develop into females; the rest develop into males. 

 Those which develop into females will often show a 

 primary maleness which may manifest itself in the pro- 

 duction of sperm or of other secondary male sexual 

 characters. This tendency is stronger the longer the 



