CROWDING AND SEX DETERMINATION 297 



Of 4 others tested after 75 days, 3 were definitely females, while the 

 other, the smallest of the lot, which had been closely associated 

 with a larger specimen, was a fully developed male. 



In contrast with the preceding experience, when known neuters 

 were transferred to a colony containing larger individuals, there was 

 a definite development in the male direction regardless of the sex 

 of the large individuals. In one experiment Crepidula examined 

 and found to be in a neutral stage were transferred to the vicinity of 

 large females. At the end of 34 days the specimens were sectioned 

 for microscopic examination. Of 27 individuals so treated, 18 had 

 adult testis, 6 were in the spermatid stage, 2 had spermatocytes, i 

 showed spermatogonia only, while none of the entire lot remained 

 in the sexually indifferent stage and none had developed in the 

 female direction. 



When small males were placed near larger males in an attempt to 

 find the effect of such proximity upon the continuation of the male 

 phase, it was found that fewer small males underwent degeneration 

 of the sexual organs than when they were completely separated from 

 larger animals. More showed degeneration than if they were in the 

 presence of large females. Even a large animal with a degenerate 

 testis will give a stimulus toward male development to a smaller 

 one near-by. An immature female, formerly a male, will also have 

 the same effect. After degeneration of the testis and the accompany- 

 ing secondary organs, these may be regenerated if the individual 

 comes under the influence of a larger animal; but the largest animal 

 in an experimentally arranged colony never shows regeneration of 

 the testis following degeneration. There is no evidence that a small- 

 er animal can affect a larger one or that a group of smaller ones can 

 have this effect, although the last point does not seem to have been 

 specifically investigated by Gould. 



A mature or nearly mature female does not undergo degeneration 

 of sexual organs however placed ; but if a partially developed female 

 is placed near a large, mature female, the female sex organs degen- 

 erate and the male organs develop. 



The degeneration of the testis in the absence of a large female 

 is not due to the lack of opportunity for copulation, for degeneration 



