Influence of the Nervous System 671 
makes its appearance. Externally the regenerated tail can be 
distinguished from the stump by (1) its color—tt lacks the brillant 
spots characteristic of D. viridescens, and by a general dull slate 
background not typical of the rest of the animal; (2), its size—it 
is sen thinner and smaller dorso-ventrally, and a distinct 
depression marks the limit of the old tail, at least in the early 
stages; (3) its softness due to lack of ossification of the skeleton. 
Internally the regenerated area is characterized by the presence 
of embryonic or incompletely developed musculature—especially in 
the early stages—by the cartilaginous skeleton, by the large masses 
of undifferentiated tissue in the dorsal and ventral parts of the tail. 
It is seen that both externally and internally there is a more or less 
clearly marked division between the old and the regenerated areas, 
and the distinction becomes less as the new parts grow longer. 
The nerve cord, however, rarely shows such a clear separation of 
the two parts. 
The removal of the cord was a relatively simple matter as 
already described, but the complete extraction of the spinal ganglia 
was difficult and uncertain. I shall first deal with motor influences, 
leaving for later consideration the study of the sensory stimuli. 
A number of normal tails were first examined in serial sections. 
A reconstruction of sections of the tail showing the approximate 
distribution of the nerves is partly seen in Fig. 1. The innerva- 
tion is quite the same throughout the tail. The spinal column 
extends to the very end of the tail. Each vertebra encloses a pair 
of spinal or dorsal ganglia, through which the dorsal and ventral 
roots pass. ‘These unite beyond only to branch again, one passing 
dorsally, the other ventrally. Many smaller branches are. given 
off from these. The point of especial importance, however, lies 
in the fact that the nerve fibers extend only into the region of the 
next two or three vertebra, and not further distally. In other words, 
the nerves at any level have their origin on in oneor more of 
the next three segments, 1. e., about 4 to 44 mm. anterior to any 
level. It follows that the reitoval of the source of motor impulses 
from the amputated end depends on the complete destruction of 
the nerve cord for at least 5 mm. anterior to the cut end. 
