SPINAL CORD REGENERATION. I 493 
deprives the cord of access from the front of the body to the 
hinder end. In such cases also, the regenerative processes in the 
cord begin, but cease after a time. The elevation of one end of 
the notochord is usually accompanied by bodily deformity of 
the type generally found in these embryos and, in consequence, 
the mechanical obstruction is combined with excessive separation 
of the cord stumps. Either factor may, by itself, prevent re- 
establishment of the continuity of the cord. 
In older animals, especially adults, the connective tissue be- 
comes a factor in preventing regeneration by strangling the cord. 
This has been noted by several writers. In the embryos under 
discussion, however, the mesenchyme plays no such rdle. 
SUMMARY 
1. Regenerative processes in the spinal cord severed in the 
cervical region in frog embryos at the stage following the closure 
of the neural folds will reéstablish its anatomical and physiologi- 
eal continuity under favorable circumstances. 
2. When the cut ends of the spinal cord are brought into 
contact with one another, healing per primam results. When 
the wound surfaces are not apposed, reunion and return to 
nearly normal form and structure are brought about by the 
following steps: (a) the development of nerve fibers from the 
motor cells of each segment of the cord, (b) the growth of sensory 
axones from the cut surface of the posterior stump, (c), the out- 
growth of fibers from the epithelial cells of the canalis centralis 
of either end of the cord, (d) the wandering of neuroblasts into 
the fibrous net between the cut ends from both stumps of the 
cord, and (e) the elongation of both ends of the spinal cord to- 
ward each other by the proliferation of epithelial cells of the 
canalis centralis and the consequent lengthening of the canal 
itself. 
3. The elements entering into the regenerated portion of the 
spinal cord are derived entirely from the original cord. The 
surrounding connective tissue plays no réle in the rehealing 
process. The epidermis probably takes no part in it. 
