SPINAL CORD REGENERATION. II 439 
by the peculiar spur remaining from the dorsal fin upon the 
back (fig. 1, F). 
Responses of embryos with unfused spinal cords to tactile stumu- 
lation. Embryos in which the middle section of the spinal cord 
has been reversed and in which the wounds have not been per- 
mitted to fuse begin to respond to tactile stimulation approxi- 
mately at the same time as those in which primary fusion of the 
wound surfaces has taken place. Those operated in the open 
neural fold stage seldom show any reaction to tactile stimulation 
until about thirty-six hours after operation, while those which 
were operated in the beginning tail bud stage begin to respond 
about twelve hours earlier. This is undoubtedly due to the 
greater differentiation of the nervous system at the time of 
operation in the latter. The majority of the stimulations of 
these embryos was carefully watched under the binocular, as 
the regions are small and consequently their movements diffi- 
cult to perceive accurately with the naked eye. In these em- 
bryos, of course, there is complete anatomical separation at the 
sites of the two wounds in the earlier stages. Care was taken 
that the myotomes were not directly stimulated. 
The first response to tactile stimulation is exhibited by the 
middle piece, irrespective of the age at which the embryos were 
operated upon. ‘The earliest visible response is a slight twitch- 
ing of the myotomes on the same side as that which receives the 
stimulation. Within two hours after this, these embryos ex- 
hibit a marked contraction of the middle piece into a well 
defined are, the concavity of which is toward the side stimulated. 
Attempts were made to carefully stimulate only minute areas 
of the middle piece, but contractions of the entire segment resulted 
in every case. It was impossible to demonstrate any greater 
sensitivity to tactile stimulation of one end of the reversed 
piece over the other, or of any tendency on the part of the 
myotomes of either end to contract more vigorously than those 
of the other. The reversed middle piece reacted throughout as 
a unit. If the cut passed through the caudal extremity of the 
medulla, the head remained inert for some time after the middle 
piece had begun to respond. If the cut, on the other hand, 
