192 G. E. COGHILL 



skin, whereas the threshold of stimulation at such a time is much 

 lower when the tip of the hair is moved gently over the surface 

 of the skin. This is a conspicuous feature in the behavior of these 

 embryos in the early period and is obviously to be accounted for 

 upon the principle of summation of subminimal stimuli or alliance 

 between reflexes. In the movement of the hair over the surface 

 of the skin several giant ganglion cells, such as illustrated in 

 figure 18, would be excited, or several endings of a single neurone 

 such as that illustrated in figure 24. In case the stimulus excites 

 more than one neurone, summation would seem to occur in the 

 associative center; in case there is excitation only of several end- 

 ings of a single neurone the summation phenomena could be re- 

 ferred only to the individual ganglion cell. It is reasonable to 

 infer, however, that the movement of the hair upon the skin in the 

 direction of the longitudinal axis of the embryo would stimulate 

 numerous endings of several neurones, and that, therefore, there 

 are exhibited here in this primitive reflex mechanism both the 

 summation stimuli in the giant ganglion cells and the alliance be- 

 tween afferent stimuli in the associative center. There is nothing 

 in this experiment upon which positively to base a differentiation 

 between summation in the peripheral neurone and alliance of 

 reflexes in the associative center; but since an experiment to be 

 described later demonstrates conclusively the phenomena of 

 antagonism between reflexes in these embryos there is strong pre- 

 sumption in favor of the idea that there is here effected an alliance 

 between neurones that have been peripherally stimulated at 

 intervals, for as the hair moves over the skin there is extension of 

 both time and area of stimulation. 



(2) The action of hydrochloric acid as a stimulating agent. 

 Sheldon's work upon the reaction of the dogfish to chemical 

 stimuli ('09) called my attention to the desirability of extending 

 my studies on amphibian embryos to their reaction to substances 

 in solution, with a view to correlating the action of chemical 

 stimuli with definite structural elements in the nervous system. 

 During several seasons, therefore, my attention has been given in 

 part to experimentation upon this phase of the problem. 



