436 _ DAVENPORT HOOKER 
In those embryos in which healing has not occurred no sign of 
a reversal of polarity ever presents itself. For any evidence 
that later adaptation produces anything similar to reversal of 
polarity in the older stages of the healed embryos, we must turn 
to a consideration of their behavior. 
Responses of embryos with primarily fused cords to tactile 
stimulation. As previously noted, the embryos were stimulated 
with a human hair according to the method of Coghill. Em- 
bryos in which a portion of the spinal cord had been reversed 
and in which primary fusion had taken place began to exhibit 
responses to tactile stimulation from twenty-four to thirty-six 
hours after operation. In the case of those embryos operated 
when from 2.5 to 3.5 mm. in length reactions to stimulation 
appeared in the latter part of this period in most cases, while 
those operated at a later stage, averaging 5.5 mm. in length, 
usually reacted twenty-four hours after operation. In every case 
the first reaction to appear was a decided bending of the head 
toward the side stimulated. But a single response followed each 
stimulation. Inasmuch as Coghill (09) has described this type 
of reaction as occurring only occasionally and very irregularly in 
Amblystoma, a large series of normal frog embryos were carefully 
stimulated with a human hair in order to obtain information in 
regard to the frequency of its occurrence in that animal. The 
results of these experiments demonstrated quite conclusively 
that in the frog embryo a contraction of the myotomes of the 
same side as that which receives the stimulation is the first 
normal type of reaction to tactile stimuli. It is of course evi- 
dent that direct stimulation of the myotomes would result in a 
similar type of response. Great care was therefore taken to be 
sure that the pressure of the hair upon the embryo was not 
sufficient to directly stimulate the myotomes. ‘To render con- 
trol of this point more satisfactory all stimulation was per- 
formed while the embryo was being watched with a Zeiss binoc- 
ular microscope. It was found that by gently touching the 
surface of the embryo with a slow stroke-like movement of the 
hair at a point just ventral to the center of the myotomes, with- 
out the slightest indentation of the skin of the tadpole having 
