ioo SCIENCE PROGRESS. 



the right way, remains perfectly still ; and on reversal 

 of the current the behaviour of the two individuals is 

 reversed, e da capo. 



So that, with Hermann, we may be satisfied to conclude 

 that the essential fact upon which the galvanotropic effect 

 turns is that current down the animal has a quieting- action, 

 current up the animal an exciting action. 



The presence of the spinal cord is an indispensable 

 condition of the success of the experiment. In this trough 

 I have now placed two tadpole tails, one cut off near the 

 head, the other about half-way down ; the first includes a 

 bit of spinal cord, the second does not ; both tails lie along 

 the line of current, which, as you see, causes the first tail to 

 tremble when it passes in an upward direction. There is 

 not a tremor to be seen when the current passes in an 

 opposite direction, i.e., towards the tip. The other tail, 

 viz., that without spinal cord, remains motionless with both 

 directions of current. 



We are now properly prepared to see more clearly than 

 would otherwise have been the case the rationale of the 

 experiment in its first and most striking form, as first 

 witnessed and described by Hermann. 



This flat trough (ioo x 75 mm.), about as large as will 

 fit the lantern, contains a number of fresh tadpoles, moving 

 more or less leisurely and jostling each other in all directions. 

 I suddenly close a key, sending through the water and the 

 tadpoles a current of about 30. The commotion is amazing, 

 the tadpole community seems to have gone mad, a writhing 

 mass is all that can be distinguished ; but the disturbance 

 does not take long to subside, and now all the tadpoles are 

 fixed as if at attention looking one way, heads to anode, 

 viz., traversed by a current from head to tail, stroked down 

 the right way. This is galvanotropism in so far as it is an 

 orientation of the body under the influence of the galvanic 

 current, but it is hardly galvanotropism of a simple and 

 fundamental character analogous with the galvanotropism 

 of protozoa, or the heliotropism of plants. It is a reflex 

 adjustment of the animal to its environment effected through 

 the agency of the nervous system ; the tadpoles have ex- 



