Influence of the Nervous System 685 
injury to surrounding tissues and with the extent of the nerve cord 
removed. The cord may regenerate at all levels, except near the 
lumbar region, regardless whether 2, 4 or 14 mm., etc., were 
removed. We may omit the details concerning the regenerating 
cord in order to emphasize the fact that when a few millimeters 
were abstracted, the new cord soon grew down to the amputated 
level and a new tail appeared. In 6 animals however the cord had 
not grown to this level, though the elapsed time between operation 
and preservation was two to three times greater than that required 
for the tail to regenerate in control animals, and not one of these 
had developed a tail. Likewise in the series wherein 7 to 20 mm. 
of the cord were destroyed, a new cord grew in many animals 
towards the amputated end, but z7 no case did the tail appear until 
the cord actually reached the end. In one animal at least this had 
taken place and a new tail developed. 
It is almost certain that after 2, 3 or 4 mm. of the cord are 
removed, subsequent degeneration prevents any motor stimuli from 
reaching the cut surface, at least during the early stages of regener- 
ation of the new tail. There is also very good reason for believing 
that even after ordinary amputation the degenerative changes in 
the cord anteriorly prevents innervation of the cut surface. 
The determining factor in the formation of a tail is not whether 
the cut surface is or is not supplied with functional motor nerves, 
but whether the nerve cord is present at the amputated end. If it 
is not present neither the absence nor the presence of functional 
motor and sensory stimuli suffices to induce regeneration. 
The problem is further complicated by the presence of another 
factor, that determines whether regeneration shall or shall not 
take place. Ina number of the animals the operation had been so 
severe that not only the cord but practically the whole of the skele- 
tal axis near the end was destroyed. In every such instance 
regeneration was inhibited even though the nerve cord was present 
at the end. As soon however as a new skeleton grew to the end, 
and provided that the cord was also there, the new tail began to 
develop. 
The matter was further tested by preventing the nerve cord from 
reaching the amputated surface, yet at the same time practically 
