194 G. E. COGHILL 



a latency period frequently followed stimulation. In embryos of 

 the early-flexure stage this period was noted in some instances as 

 being ten to twenty-five seconds in duration. 



In embryos of the early-flexure stage reaction followed stim- 

 ulation on the caudal portion of the trunk as well as on the head, 

 and the movements were of the same nature as if they had been 

 stimulated by tactile means. Furthermore, in case of transection 

 of embryos it is found that caudal portions that respond to tactile 

 stimulation respond also to chemical stimulation, and caudal 

 portions that do not respond to tactile stimulation do not re- 

 spond to chemical stimulation. In other words, in development 

 and in mutilations response to chemical stimulation comes and 

 goes hand in hand with response to tactile . stimulation, and the 

 two forms of stimuli excite the same forms of response. These 

 facts create a strong presumption against the idea that there are 

 different mechanisms involved in the reactions to the different 

 forms of stimulation. 



By the use of a spray it is obviously impossible to determine 

 the threshold of stimulation exactly, because of the increasing 

 dilution of the acid as it diffuses through the water. To study 

 this point more carefully and study comparatively the action of 

 different concentrations of the acid, embryos were immersed in 

 various solutions and their activity observed. The following 

 experiment, which has been repeated with various modifications, 

 may be accepted as representative of the results of this method 

 of study. 



Four Amblystoma embryos of the early-swimming stage were 

 immersed in similar dishes containing the following solutions: 

 (a) cistern water, from the stock used for growing the embryos 

 in the laboratory; (b) distilled water which had been aerated by 

 pouring from one dish to another repeatedly; (c) HCl n/2000; 

 (d) HCl n/3000; (e) HCl n/4000. The distilled water used in 

 diluting the normal solution of the acid had also been aerated in 

 the manner mentioned. 



At the expiration of five minutes the greatest activity was shown 

 by the embryos in the dish with HCl n/2000. Two minutes later 

 there was slight activity among the embryos in HCl n/4000, 



