Vol. XXXIV. June, 1918. No. 6. 



BIOLOGICAL BULLETIN 



EFFECT OF ENVIRONMENT UPON INHERITED 

 CHARACTERS OF H YD ATI N A SENT A. 



A. FRANKLIN SHULL, 

 DEPARTMENT OF ZOOLOGY, UNIVERSITY OF MICHIGAN. 



INTRODUCTION. 



Several years ago it was discovered (Shull, 1915) that two dis- 

 tinct parthenogenetic lines of the rotifer Hydatina senta, one 

 from England, the other from Nebraska, differed in certain 

 physiological (and perhaps structural) characters: (i) the Eng- 

 lish line laid smaller eggs than the Nebraska line; (2) the English 

 line habitually laid a large percentage of its eggs attached to the 

 surface film of the water, while the Nebraska line laid most of its 

 eggs at the bottom or sides of the vessel; (3) the eggs of the Eng- 

 lish line required longer to develop than did those of the Nebraska 

 line; (4) and when the rotifers were killed in Bouin's fluid the 

 foot was seldom, or only slightly, retracted in the English line, 

 but considerably retracted in the Nebraska line. 



When crosses were effected between these two lines, the FI 

 lines and F 2 lines were all indistinguishable from the English 

 line in all the above-named characteristics. It seemed as if 

 segregation and recombination had failed, and that in some way 

 the four characters were rigidly associated one with another. 



At first it was regarded as possible that the four characters 

 were not really distinct, but were different manifestations of a 

 single (physiological) character. That character might have 

 been a greater permeability of the cells in one line than in the 

 other. Thus, if the Nebraska line were more permeable to 

 oxygen, the increased metabolism might make its eggs larger. 

 For the same reason the eggs of the Nebraska line might develop 

 in less time. In like manner it might be that the Nebraska 

 rotifers, able to get the required amount of oxygen at the bottom 



335 



