THE NERVOUS SYSTEM ■ OF AMPHIBIA 199 



highly differentiated and innervated surfaces in the oral cavity 

 of man. But this is not all, a distinctly perceptible reaction 

 of the ectoderm cells of these embryos has been observed in HCl 

 n/10,000. 



From these and from numerous other experiments with hydro- 

 chloric acid as a stimulating agent, the following conclusions have 

 been drawn: 



1. The ciliated cutaneous epithelial cells of young Amblystoma 

 embryos react directly to exceedingly dilute solution of hydro- 

 chloric acid without the intervention of a nervous system or 

 nervous connection of any kind. 



2. No degree of concentration of the acid can be found which 

 will excite muscular response without causing destructive processes 

 in the skin. 



3. There is a distinct correlation between the nature of the 

 muscular response and the injurious action of the acid which is 

 used as a stimulating agent. 



4. There is no evidence of a normal irritability of the skin to 

 acid. 



5. If there should prove to be such a normal irritability, it 

 must necessarily act upon the same reflex mechanism as that 

 through which reaction to tactile stimulation takes place. 



c. The proprioceptive field 



The anatomical part of this paper has dealt in detail with the 

 endings of the giant ganglion cells upon the myotomes. That 

 such endings are in fact sensory is demonstrable experimentally. 



It has been explained in various connections that, during 

 a certain period of development, particularly during that period 

 which is characterized by the coiled-reaction, these embryos 

 move almost constantly away from the side touched. When the 

 embryo reaches the swimming stage, however, the movements 

 become more irregular in direction relative to the side stimulated, 

 and with further development every suggestion of this law of 

 response that prevailed in the earlier period disappears. If, how- 

 ever, an embryo of the advanced swimming stage, which has 



