The Determination of Sex 369 



ditism is found, although in only one of these, the flat-worms, is 

 it the almost exclusive condition. 



It would be interesting to take up these groups in turn and 

 examine the relation of the two kinds of sexual individuals, but 

 I shall select only one group in which the relations are especially 

 instructive, although exceptional. 



In the group of Nematodes, or round worms, we find a curious 

 case of alternation of two kinds of sexual individuals. Ascaris 

 nigrovenosa is an hermaphrodite form found in the lungs of 

 frogs. The eggs pass through the digestive tract of the frog and 

 hatch as the Rhabditis form that has separate sexes. Equally 

 remarkable are the cases in which there exists an hermaph- 

 rodite form that appears to represent the female individual in 

 which secondarily spermatozoa as well as eggs have developed. 

 Maupas has studied these species with great care, and I follow 

 his description. 



Maupas points out that there have been described 34 species 

 of Nematodes in which reproduction without males has been 

 established ; of these 34 species 25 are hermaphroditic and 9 

 parthenogenetic. 1 Of the 34 species, 16 belong to one genus 

 Rhabditis. We are concerned here only with the hermaphro- 

 ditic species. These forms differ in no bodily character from 

 the females of other species with separate sexes, and differ only 

 in the ordinary specific differences/ A minute study of their 

 sexual organs at the time of maturity is necessary to detect their 

 hermaphroditic character. It is found that when the reproduc- 

 tive organ is fully formed, it functions first as a testis. The germ- 

 cells at the. anterior end of the ovary begin to divide rapidly and 

 become small spermatozoa. These are stored up in a receptacle 

 of the uterus. Later other cells, also situated at the anterior 

 end of the ovary, begin to grow larger; they store up yolk, 

 and become large eggs. They enter the uterus and become fer- 

 tilized by their "own" spermatozoa. 



1 This list Maupas says will probably be augmented, for of the 206 species 

 that are known from fresh water and from land, 85 are known only by the 

 females. 



