24 STEREOTROPISM OF THE DOGFISH 



ing object. At the same time the dorsal fins and the tail would act 

 as a steering apparatus to alter the course to a direction away from 

 the source of stimulation. This then is also a definite tropic reaction 

 and in the negative sense. 



Attention should perhaps be called to the fact that while the posi- 

 tive reaction is on the whole opposite in character to the negative 

 it is not precisely so. A weak stimulus on the right side of the snout 

 or a strong stimulus on the left side would each tend to turn the 

 ventral side to the right and cause the course to veer to the right. 

 But the mechanism is not quite the same so far as the pectorals are 

 concerned. In the positive reaction the pectorals act feebly in 

 comparison to the unpaired fins; in the negative reaction their move- 

 ment is relatively more vigorous. In both the positive and the nega- 

 tive reaction the pectoral of the stimulated side is elevated; but in 

 the one case its posterior margin is elevated less than the anterior 

 and in the other case more. It will not do then to say that the nega- 

 tive reaction differs from the positive merely in the fact that the exci- 

 tation is shunted from one side of the central nervous system to the 

 other. It is certainly not so simply diagrammatic as that. 



The Decerebrate Animal. 



My experiments on Rkitiobatus led me to expect that the destruc- 

 tion of the forebrain would have no effect on the character of the 

 stereotropic reactions of the dogfish. In a number of instances I 

 made transections of the brain, usually near the anterior margin of 

 the cerebellum, with no noticeable alteration in the responses to con- 

 tact stimuli. The following record of an experiment will serve as an 

 example : 



July 4, 1921. — Mustelus californicus, 29 inches long. 



9:20 a.m. Brain exposed and cut across at anterior margin of cerebellum. 

 Animal returned to tank, lies inert; does not right itself. 



10:30 a.m. Animal swimming about normally. Taken out and placed on board. 

 Contact reactions tested. (Table II.) 



Possible Sources of Error. 



Certain possible sources of error were considered and should be 

 mentioned. 



