486 DAVENPORT HOOKER 
taken by it. In tadpole VIII 73, this deviation takes the form 
of a sharp, lateral V-shaped turn toward the left. Owing to the 
fact that the plane of section of this tadpole is not exactly per- 
pendicular, only the tip of this bend has been cut in the section 
figured. ‘ 
DISCUSSION 
It is evident from the experiments that, under favorable con- 
ditions, the spinal cord of frog embryos will regenerate after 
having been completely severed in the cervical region during the 
stage following the closure of the neural folds. The primary 
reunion is effected by the development of nerve fibers which 
bridge the gap and establish the physiological continuity of the 
cord. Such rehealing was reported as far back as 1849 by Brown- 
Séquard. ‘Two months after he had completely severed the spinal 
cord in the thoracic region of 2 adult pigeons, he found white 
fibers (‘filets blancs’) traversing the tissue which united the cut 
ends of the cord. These were identified by M. Follin (Brown- 
Séquard, 750) as nerve fibers, after the microscopic examination 
of the regenerated area in a similarly operated pigeon killed 3 
months after operation. Harrison (’10) has observed the begin- 
ning of the same process as that described in this paper. After 
removal of relatively large portions of the spinal cord from frog 
embryos in the closed neural fold stage, nerve fibers grew from 
each of the cut surfaces toward the other. Though these nerves 
did not reach their supposed objective points before the embryos 
were killed (4 days after operation), it is evident that their out- 
growth represents an attempt st restoration of the continuity 
of the cord. 
The role played by both ends of the severed cord in the re- 
generative processes is significant. It was found that the ante- 
rior piece of the cord initiated the various steps in the restoration 
somewhat in advance of the posterior. This is in full accord 
with the work of Coghill (13 and ’14), who has shown that 
differentiation in the spinal cord begins in its rostral end and 
proceeds caudally. Nevertheless, the posterior end does con- 
tribute elements to the regenerated area. Opposed to this 
