SPINAL CORD REGENERATION. I 483 
in length. During their growth the body had rounded out and 
the portion of the dorsal fin anterior to the cut had disappeared. 
The site of the cut was marked only by a slight crease in the 
dorsal surface of the body. The behavior of these individuals 
was In every way normal. 
Sections of larvae killed at this time demonstrate that the 
spinal cord and notochord had been severed in all cases. The 
apparently normal individuals exhibit the final steps in the 
complete restoration of the anatomical continuity of the cord. 
In the tadpoles showing the stage following the one last de- 
scribed, the two cut ends of the cord are united by a dense mass 
of fibers. They are of three types: 1, neuraxes, rather broad and 
longitudinally striated by reason of the neurofibrillae which 
they contain; 2, connective tissue fibers, granular in appearance; 
and 3, a large number of the fine, branched fibers arising from the 
epithelial cells of the canalis centralis. The majority of these 
last appear to come from the anterior stump of the cord, though 
many arise from the posterior. They are by far the most numer- 
ous of the three types of fibers and bind the motor and sensory 
connections into one heavy cable. Their growth into the fiber 
mass connecting the two ends of the cord is accompanied by the 
partial exclusion of the connective tissue fibers from it. This 
exclusion is complete where the neuroblasts have migrated into 
the network of neuraxes and ependymal fibers. Where this has 
occurred, the connective tissue forms an enveloping membrane 
over the outside of the neural elements, easily distinguishable 
from them by the granular nature of the cytoplasm and the 
smaller size of the nuclei. 
Though the great bulk of the regenerated area is composed 
of the two types of neural cell processes, an increase in the number 
of neuroblasts which have migrated out from the cord is to be 
noted. In this stage, the posterior stump has begun to contrib- . 
ute these cells. Further evidence is presented by these tad- 
poles that the two stumps of the cord are approaching one an- 
other by a process of subapical growth, accompanied by the 
progressive canalization of the group of epithelial cells about 
the ends of the canalis centralis. Positive proof that the central 
