EFFECT OF ENVIRONMENT UPON HYDATINA SENTA. 349 



If the rotifers were kept at a low temperature, their eggs were 

 laid more largely at the bottom. This may be due to the greater 

 concentration of oxygen in the water at low temperature, so that 

 it is unnecessary to come to the surface so much; or to the low 

 metabolism and hence low oxygen requirement of the animals; 

 or to both of these reasons. 



Excess of oxygen increased the size of the parthenogenetic 

 eggs only very slightly, or not at all. Excess of oxygen decreased 

 the time of egg development only very slightly, or not at all. 

 It may have made the time of development a trifle more uniform, 

 though this is hardly proven. 



Rotifers kept in oxygenated water showed, when killed in 

 Bouin's fluid, a trifle less contraction of the foot muscles than did 

 other rotifers, though the difference was too small to constitute 

 a proof of the action of oxygen. 



Since the place of laying the eggs (surface or bottom) was so 

 much more greatly affected by oxygen than were the other three 

 characters tested, it would seem rather improbable even if the 

 genetic results (Shull, 1915) had permitted such an assumption, 

 that the several characters in which the English and Nebraska 

 lines differ as described in the introduction to this paper, were 

 really but a single character with several manifestations. The 

 results described in this paper, therefore, harmonize with the 

 genetic results previously obtained. 



Freezing fertilized eggs that had remained unhatched for five 

 to seven weeks did not induce any of them to hatch later. 

 However, when eggs were kept from the time of laying at a low 

 and variable temperature above freezing, more of them hatched 

 than when kept at room temperature. 



When fertilized eggs were kept in oxygenated water, a some- 

 what smaller proportion of them hatched than in untreated 

 water. 



Cross-fertilized eggs which had remained for seven weeks 

 under ordinary conditions without hatching were dried over 

 night and then remoistened. A considerable number of them 

 were thereby caused to hatch. Inbred eggs, however, when 

 tested in the same way, did not respond to drying. Even inbred 

 eggs that were dried shortly after they were laid (not merely 



