250 CASWELL GRAVE 



5.30 P.M. A tadpole sinking motionless, tail upward, the body 

 revolving slowly on its long axis, became active and swam to the top. 



5.45 P.M. A few tadpoles actively swimming in the upper half of 

 the cylinder, some near the bottom, many have become attached. 



6.00 P.M. All tadpoles attached, a few at and near the top, some to 

 the side, but the greater number to the bottom or the edge of the bot- 

 tom on the side of the cylinder least illuminated. 



Tadpoles are alternately active and quiescent during the 

 free-swimming period and, as may be noted from observations 

 just recorded, slowly sink during their quiescent periods, and are 

 thus carried passively from points at various levels into deeper 

 strata of water or even to the bottom. This fact at once sug- 

 gests the possibility that the proportionately large number of 

 tadpoles which become attached to the bottom may include 

 many that cannot be said to have migrated from the upper 

 surface in consequence of a positive response to gravity. To 

 test this possibility, several experiments were carried out which 

 are similar to those just described in every way except that the 

 cylinder was inclined 45° from the vertical. In a cylinder thus 

 inclined tadpoles which come to the end of their free-swimming 

 period or become quiescent while in the upper strata of water, 

 or at any level higher than 8 cm. above the bottom, can sink a 

 short distance only before reaching and becoming attached to 

 the side. The bottom will be reached by those only which 

 actively seek it out. 



The results of these experiments, recorded in columns 9, 10, 

 and 11 of the table, are practically identical with those in which 

 the cylinder stood upright, recorded in columns 7 and 8, showing 

 rather conclusively that tadpoles become attached to surfaces 

 situated considerable distances below the top of the water and 

 to the bottom, not because they passively sink from the higher 

 levels, but in consequence of a definite positive response to 

 gravity. 



The cause or causes of the reversal in the geotropic response 

 of tadpoles from negative to positive have not been discovered. 

 The suggestion that a change in the position or orientation of 

 the statolith cell in the wall of the sensory vesicle possibly takes 

 place which may be correlated in some way with the change in 



