CROWDING AND SEX DETERMINATION 295 



The method of effecting male determination in the case of the 

 Bonellia larvae appears not unlike that which effects the transfor- 

 mation of a zygotically determined female calf embryo which is in 

 blood-stream connection wath a twin of the opposite sex. The well- 

 known work of F. R. Lillie (191 7) has shown that under these con- 

 ditions a transformation in the male direction takes place, due to 

 the effect of a hormone produced by the male upon the organs of 

 the developing female. 



SEX IN CREPIDULA 



In the gasteropod mollusk, Crepidula plana, Gould (1917) has 

 shown that the sexual life of adults of this marine snail may be divid- 

 ed into (a) the male phase, {b) the transitional phase, and (c) the 

 female phase. In other words, these animals are protandric her- 

 maphrodites with the opposed sexual phases completely separated. 

 The development of the male condition does not always take place 

 at the same stage with respect either to age or to size of the individ- 

 ual, and at times may be omitted entirely. 



Male development occurs, if at all, during the early life of the 

 adult. The growth of the animal during this period is highly vari- 

 able, depending, among other factors, on the amount of movement, 

 the extent of available space, and the season of the year. Clearly 

 distinguishable primordial male and female germ cells are both to 

 be found in the gonads of these animals from the postlarval stage 

 up to the time of complete female development, and can be dis- 

 tinguished from each other under the microscope. During the period 

 of transformation from the male to the female phases, the testis 

 degenerates; and finally all the primordial male germ cells disappear 

 as the gonad becomes reduced in size. With the assumption of the 

 female phase the gonad again enlarges. 



The same duct serves for the passage of the sperm in the earlier 

 phase and the eggs in the later, but it undergoes marked changes in 

 the transition. Secondary male characteristics, such as the penis, 

 the sperm groove, and the seminal vesicles, are present only when 

 the testis is developed ; they appear when the testis appears and dis- 

 appear when it degenerates. 



