174 J: M. D. OLMSTED 
of the original lateral margin must serve as a tail, or a tail is 
unable to appear on this ‘posterior’ end because there is no new 
tissue at this region to form one. The former of these two possi- 
bilities was shown by the experiments illustrated in figure 14 
not to have been realized under seemingly favorable conditions. 
The planarian did not suffer a lateral margin to serve as a tail, 
but produced a new tail, or at least added enough new material 
to make the old lateral margin again lateral in the new worm. 
To test the second possibility, the ‘posterior’ ends of four of 
these triangular pieces whose heads had already been developed 
and polarity established, were either cut off or so wounded that 
new tissue appeared there. In all four cases the wound healed 
over with only enough new material to repair the injury. No 
sign of a projection which could be taken for a tail was to be 
seen. In these triangular pieces the new material which at 
first appears along the original posterior edge may be potentially 
tail-forming substance, but on account of the overwhelming 
tendency toward the production of a head, this material moves 
anteriorly, becomes a part of the head, and all possibility of 
regenerating a tail is lost. 
There is a possibility that the peculiar condition of the nervous 
system in these triangular pieces may inhibit the regeneration 
of a tail. Had regeneration proceeded along the same lines as 
in large triangles whose angle at the vertex is more than 90°, a tail 
would have been formed in the new tissue just posterior to the 
end of the old nerve cord (or its prolongation). In this region 
the two nerve cords, the old and the new, run parallel to each 
other, as do the two old nerve cords in figure 1., But what actu- 
ally happens is that the old and new nerve cords in such pieces 
never lie side by side, but become joined end to end. Figures 8 
to 11 show the nervous system of these pieces so completed that 
there is no place in the scheme for a tail. It is more probable, 
however, that the factors which determine that the nervous 
system shall be an unbroken ring are the very ones which also 
determine that there shall be no regeneration of a tail. The 
influence of the nervous system on regeneration is, however, an 
unsettled question. 
