96 SEX INHERITANCE 



sex but depend on a difference in their medium. If 

 such an environment can be discovered it would be 

 sex determining in the same sense in which the term 

 is here employed when the sex differentiators are 

 hereditary factors. 



Sex determination in the Gephyrean worm Bonellia 

 is a case in point. The female is a large oval worm 

 with a long proboscis. The male is very small and 

 degenerate and lives as a parasite on the proboscis 

 of the female. The development has recently been 

 studied by Baltzer. He finds that if the young 

 Bonellia embryos are put into an aquarium in which 

 old females are present, they settle down on the 

 proboscis of the female and degenerate into males. 

 If, on the contrary, the young embryos are kept by 

 themselves they pass through an indifferent stage 

 but later differentiate into female worms. Whether 

 a male or a female develops from an egg depends 

 here on whether at a certain stage the embryo comes 

 under the influence of the proboscis of a female or 

 fails to do so. Some secretion from the proboscis 

 may be the differentiator in such a case. It is clear 

 that here it is environment that determines the 

 sex of the individual. The evidence suggests that 

 the male organs develop first in the presence of a 

 certain secretion from the proboscis of the female 

 which also serves to arrest the animals in this stage. 

 If, however, the animal fails to meet with these 

 conditions, it usually ceases for a time to develop 

 and fails to produce the male organs. Later it 

 starts once more to go forward and develops the 



