SPINAL CORD REGENERATION. I 485 
side of this point it presents certain irregularities in its course. 
The unregenerated canalis centralis is regular in outline, some- 
what oval in cross section and lies in the center of the cord. 
The regenerated canal is rather tortuous in its course, varies 
from nearly a circle to a very narrow, parallel-sided slit in cross 
section and does not occupy a constant position in the cord. 
It is, however, single. In no case has any tendency toward mul- 
tiplication of the central canal been observed in these operated 
embryos and tadpoles. 
The third indication that the cord has been severed is to be 
found in the distribution of cells and fibers in the regenerated area. 
In the uninjured portions of the cord, the cells form a definitely 
placed group about the canalis centralis, sharply delimited from 
the fiber tracts on the periphery of the cord. In the regenerated 
portion, the regular outline of the cell area is broken and the 
thickness of the area itself varies greatly in sections of the cord 
which are quite close together. In some places, the cells lie at 
the periphery of the cord. Where this is the case, they are 
arranged in no definite manner and are found among the fibers 
which normally lie on the outside of the cord. In other places, 
the bulk of the cells lies to one side of the canalis centralis, the 
opposite side being occupied chiefly by fibers. This is notably 
the case where the central canal bends laterally or ventrally from 
its normal position. Furthermore, the nearer one approaches the 
supposed point of junction of the two ends of the cord, the 
fewer become the neuroblasts which are found there. It would 
appear that the normal number of cells is not present and that 
their arrangement is disturbed by the lack of definite separation 
of the cell and fiber areas. Whether or not the number of neur- 
ones in this region would in time increase to normal is at present 
a matter of conjecture. 
The notochord also gives evidence of fea ing been cut. The 
mass of notochordal sheath tissue, which formed a prominent 
feature of the environment of the cord stumps in earlier stages, 
has here completely disappeared. The two ends of the notochord 
have fused, but the point of fusion is marked in tadpoles of this age 
by greater or less deviations from the straight course normally 
