THE NERVOUS SYSTEM OF AMPHIBIA 185 



Although the numbers tabulated above are not the results 

 of a specific effort to count the fibers of this system, yet my im- 

 pression, based upon my knowledge of the nature of all the mate- 

 rial studied and the methods of study employed, is that it may be 

 correctly inferred from them that there is no appreciable in- 

 crease in the number of peripheral fibers in the transition from 

 the non-motile to the early-flexure stage, that almost all the 

 fibers in the younger stages pass out over or in close relation to 

 the ends of the myotomes, and that, in the older stages, there 

 are more fibers passing out to the skin over the middle portions 

 of the myotomes. 



In considering the numbers of fibers observed in the older em- 

 bryos it must be kept in mind that there occurs during this period 

 a rapid development of other structures that obscure the finer re- 

 lations of the fibers. There results from this factor a much greater 

 difficulty in following the fibers or detecting their presence and, 

 to my mind, this explains why the maximum number of fibers 

 recorded for any one individual of the two older stages is smaller 

 than the maximum number found in the two younger stages. 



(5) Summary. A summary of the peripheral relations of the 

 giant ganglion cells may be outlined as follows: 



1. The giant ganglion cells innervate both skin and muscle. 



2. A single neurone of the system may innervate both skin 

 and muscle. 



3. These relations with the skin and myotomes become estab- 

 lished throughout the extent of the segmented mesoderm some 

 time before the embryo responds to tactile stimulation. 



4. In the level of the unsegmented mesoderm the spinal cord 

 and the giant ganglion cells have strong adhesions both with the 

 skin and mesoderm, the substance of the cord in restricted regions 

 becoming indistinguishable from the skin on the one hand and from 

 the mesoderm on the other. 



5. While there is no positive proof that there is an increase in 

 the number of fibers to the skin during the period under investi- 

 gation, there is clear evidence of a progressive differentiation 

 and growth of the fibers, particularly in the extension and elabor- 

 ation of the sub-epithelial plexus so that a single fiber acquires a 



