336 A. FRANKLIN SHULL. 



of the dish where the oxygen in solution was less abundant, lived 

 most of the time at the bottom and laid their eggs there ; whereas 

 the less permeable English rotifers were forced to swim to the 

 surface where dissolved oxygen was presumably more abundant, 

 and laid eggs at the surface film. And if the foot muscles of the 

 Nebraska line were more permeable to the killing fluid than were 

 those of the English line, the greater contraction of the muscles 

 of the former might thereby be explained. 



It was possible to test the correctness of the above assump- 

 tions, in part, by aitifically altering the expression of the in- 

 herited characters through changes in the environment. A num- 

 ber of experiments were performed to this end. However, before 

 they were completed, an F 3 generation was obtained in which 

 the association of the four inherited characters was broken. In 

 this and the succeeding generations each one of the four char- 

 acters was separated at least once from the others with which it 

 was associated in the original lines. 



Thus was proven that the four characters were not merely dif- 

 ferent expressions of one character. The experiments designed 

 to test their separateness or singleness were, therefore, not com- 

 pleted, and were not published. It has now become necessary, 

 however, to refer to certain of the results, and they are here 

 described in the incomplete form in which they were left. Along 

 with them are several experiments on the viability of fertilized 

 eggs, as affected by external conditions. These are of interest 

 to the experimenter from a practical standpoint, and also in 

 relation to popular ideas concerning the fertilized eggs of Clado- 

 cera. 



EXPERIMENTS. 



Effect of oxygen upon the laying of eggs at surface and bottom of 



water. 



On each of the dates named in Table I., approximately equal 

 numbers of females of Hydatina were placed in two dishes. In 

 one was placed water oxygenated by vigorously shaking it in an 

 atmosphere having a high percentage of oxygen. The dish was 

 then set, uncovered, under a bell jar in which was confined an 

 atmosphere containing the same high percentage of oxygen as 

 that with which the water was first saturated. Since the bell 



