THE NERVOUS SYSTEM OF AMPHIBIA 191 



minute quantities, but there is no suggestion of an influence from 

 this source upon behavior, or of any nervous connection that 

 could conduct stimulation from the field of the entoderm in the 

 trunk to the spinal cord. 



b. The exteroceptive field 



The exteroceptive field of the giant ganglion cells must be 

 considered with reference to tactile stimulation and chemical 

 action. 



(1) Irritahility to tactile stimuli. From the later treatment of 

 the action of soluble substances upon the skin it will be under- 

 stood that tests for tactile irritability must be made with instru- 

 ments that are chemically inert in water. As such a stimulating 

 instrument, a hair, conveniently mounted in a holder, has been 

 used in all my experiments, a method described in my earher 

 papers and adopted by Hooker ('11) to differentiate between 

 tactile stimulation of the nerves and direct stimulation of the 

 muscles. 



The anatomical part of this paper shows that the cutaneous 

 fibers from the giant ganglion cells are estabUshed in their typical 

 relations with the skin in the non-motile embryo. Accordingly, 

 it is found that when response begins, that is to say, in the early- 

 flexure stage, irritability to tactile stimulation appears regularly 

 over the trunk without any perceptible differentiation in cephalo- 

 caudal levels. In embryos of the early-flexure stage such sub- 

 epithelial fibers as are shown in figure 24 are common, and, as is 

 well known from the work of various authors which my own 

 observations confirm, the fibers of some of the giant ganghon 

 cells turn ventrad around the lateral border of the myotomes and 

 pass far towards the ventral surface of the animal. Such struc- 

 tures as these account for a comparatively regular irritabihty 

 over the entire surface of the trunk and tail bud, even to its tip. 

 And as the dorsal and ventral fins expand into thin laminae these 

 are irritable to their very margins. 



In the very early-flexure stage response is irregular and uncertain 

 when the stimulus is applied as a light touch to a single spot in the 



