THE PROBLEM OF SEX DETERMINATION. 



157 



male, and that the sexual dimorpliisni is well marked. It was obtamed 

 some three years ago from some sandy material collected in Plymouth 



Fig. \.—I)iiwphilus (jyrociliatus. Rudimentary male and full-grown female. The female 

 shows the broken nature of the ciliated bands in the head region and the 

 solcnocyte bearing nephridia. 



Sound. I introduced it subsequently into the tanks of the Plymouth 

 Laboratory, where it has since established itself, and breeds. 



Korschelt was the first to point out that a marked sexual dimorphism 

 is present in D. (qjatri^, where the male is small and rudimentary. He 

 also observed that the female laid two kinds of eggs together in one 

 capsule. Some of these were almost six times the size of the others, 

 and were destined to give rise to tlie females, while the small ones gave 

 rise to the rudimentary males. The large female eggs are laid in the 

 proportion of three to one of the small male ones. Here is apparently a 

 clear case in which we get sex determination in the ovary long before 

 fertilization. Korschelt took it for granted that the eggs were only 

 fertilized after being laid. The presence of this species of Uinophilus 

 at Plymouth aflorded me the opportunity of thoroughly investigating 



