346 



SCIENCE 



[N. S. Vol. XXXII. No. 819 



of rotifers in jars which had been standing 

 in the laboratory from four to twelve weeks 

 were examined. Lots of five to eight hundred 

 individuals were selected at random from each 

 of these jars. In some of these lots three to 

 five males were found, while in lots from other 

 jars no males at all were found. In jars of 

 new food cultures five to ten days old and 

 stocked with individuals from these old cul- 

 ture jars, sexual females constituted as high 

 as thirty per cent, of the individuals present. 



In some experiments made this su mm er in 

 which a few parthenogenetic females were 

 placed in vials containing a small green 

 flagellate, Clilamydomonas, and put into direct 

 sunlight in order that the flagellates might 

 remain active and serve as food for the roti- 

 fers, and also to aerate the culture water in 

 the vials, males appeared either on the third or 

 fourth day and on the immediately following 

 few days. Then the males disappeared 

 entirely and the parthenogenetic females 

 increased in numbers to fifteen hundred to two 

 thousand in each vial in the course of a week 

 and a half to two weeks. These experiments 

 were repeated several times with always the 

 same result — that as the culture water became 

 older the sexual females disappeared and the 

 parthenogenetic females increased in numbers. 



However, in the summer of 1909 in a pedi- 

 gree strain of rotifers supplied entirely with 

 water and food from an old culture jar, a line 

 of parthenogenetic females was produced for 

 twenty-six generations, but as the old culture 

 water became low in the jar and the liquid 

 very near the bottom was used, males 

 appeared. This food culture was about eleven 

 or twelve weeks old at this time and had been 

 producing only parthenogenetic females for 

 seven to eight weeks, and then under appa- 

 rently the same conditions, suddenly produced 

 sexual females. 



This failure of an old culture water to pro- 

 duce continuously parthenogenetic females, 

 and the high percentage of sexual females 

 found in new culture water, suggested the 

 possibility that the production of sexual 

 females might be due to the presence of defi- 

 nite chemical substances in the culture water. 



The possibility of this suggestion was strength- 

 ened by several cases of male epidemics which 

 have occurred in my cultures. 



In new horse-manure cultures at a tempera- 

 ture of 18-22° C, sexual females occur some- 

 times as high as thirty per cent., as has already 

 been stated. In May, 1909, a large culture 

 about two to three weeks old, containing roti- 

 fers, was standing in a south exposed window 

 where it received the direct sunlight for a few 

 hours each day and had its temperature thus 

 raised to 28-30° C. Several lots of large eggs 

 were selected at random from this culture dur- 

 ing a period of three to four days, and placed 

 at room temperature in some of the culture 

 water in which the eggs were laid. In some 

 of these lots of thirty to forty eggs, ninety- 

 three per cent, developed into sexual females. 

 Soon the parthenogenetic females began to 

 increase and the sexual females to decrease in 

 numbers, so that about a week later only about 

 five per cent, of the individuals in the jar were 

 sexual females. In another jar of newly made 

 culture, in June, 1909, which was under the 

 same conditions as the former jar, practically 

 the same results were obtained again. 



In November, 1909, the laboratory was 

 closed one Sunday and the steam heat left on. 

 The temperature rose to 26° C. or more. In 

 three pedigree strains of rotifers which hap- 

 pened to be subjected to this great change of 

 temperature, sexual females appeared in each 

 strain in the following generations in greater 

 numbers than they had appeared since the pre- 

 ceding May and June. It may be recalled 

 that Maupas obtained a very high percentage 

 of sexual daughter females when he subjected 

 the adult mothers to a temperature of 26-28° 

 C. Of course, when these females were at the 

 high temperature the culture liquid in which 

 the females were living was at the same tem- 

 perature. 



From a consideration of these general obser- 

 vations, it is conceivable that in newly made 

 cultures of horse manure and water during the 

 great chemical changes that are taking place 

 in the decomposition that occurs during the 

 first two weeks, definite but transitory 

 chemical compounds are formed which so act 



