THE LIFE HISTORY OF PLANARIA VELATA. 143 



and eventually fragmented and encysted in the usual manner. It 

 is seen from these experiments that low temperature retards the 

 disappearance of sexual structures once they are developed. We 

 may now consider the possibility of experimentally developing 

 sexual worms from an asexual stock by the manipulation of the 

 temperature to which the worms are subjected. 



That there might be no question of a genetic difference of 

 strains producing sexual and asexual worms, stocks having their 

 origin from cysts have been taken as the starting point of these 

 experiments. Such worms when fed at room temperature have 

 invariably undergone a period of asexual growth and eventually 

 encysted without any suggestion of approaching sexuality. At 

 temperatures ranging from 4 to 12 the result is the same if the 

 worms are placed at that temperature soon after emergence from 

 cysts and the temperature is maintained fairly constant through- 

 out the period of growth. As might be expected growth at low 

 temperatures is much less rapid and the worms attain a much 

 greater size before encystment. These experiments show that 

 sexuality is not to be expected at either high or low temperatures 

 as long as the temperature is constant throughout the life of the 

 animals. 



Asexual worms raised to a length of ten or twelve millimeters 

 at room temperature and then subjected to a temperature of S-io 

 have repeatedly been found to show indications of approaching 

 sexuality. In such worms ovaries and testes in advanced stages 

 of gametogenesis have been found within two weeks of the time 

 of the change in temperature. Atrial organs have been practically 

 completely developed. In all of these cases the worms have been 

 so small that complete sexual development was not expected. 

 In none of the experiments resulting in the development of sexual 

 worms have the animals been fed during the time while the sex 

 organs were differentiating. It has been observed that with the 

 resumption of feeding there is at first a slight further development 

 of the sex organs that is followed by their degeneration and the 

 return of the worms to the asexual condition. A change from 

 room temperatures to temperatures of from 3 to 8 has not been 

 found effective in bringing about sexual development. Such 

 experimental stocks have continued slow asexual growth when fed 



