The Reaction to Tactile Stimuli 
253 
the asymmetry is the same with reference to stimulation on the tail 
I bud as it is with reference to stimulation on the head, and speci- 
I mens that are asymmetrical in one respect are so also in the other. 
! The structural basis for a regular movement of the head away 
I from the side touched must obviously lie in the ascendency of the 
I descending tracts which decussate in the cephalic part of the cen- 
i tral nervous system over the uncrossed long tracts which descend 
I into the cord. In comparing this condition with the response to 
' stimulation on the tail bud, it should be remembered that the path 
I from n. trigeminus or n. vagus to the opposite musculature of the 
I cephalic part of the trunk is through the descending axones of these 
: nerves within the central system, while the path from the caudal 
I nerves to the same musculature is through the ascending axones 
i of the afferent nerves. This factor will be best considered in con- 
nection with the account of reaction to touch on the tail bud. 
The most difficult phase of the problem to deal with by way of 
anatomical- inference or in the framing of a working hypothesis 
from the point of view of anatomy is the occasional response 
directed towards the side touched and the period of irregularity in 
response that precedes the period of regular movement away from 
the side touched. It is possible that in such cases the impulse 
passes directly to the centers of synapse with the effectors of the 
opposite side and, in case these centers are inactive, returns by a 
commissural path to the corresponding effectors of the same side; 
or it might be that the connection with the effectors of the same 
side is through collaterals of axones which themselves pass directly 
to the opposite side, and that, in case the opposite effectors are 
inactive the impulse may flow over into the collaterals and effect a 
connection with the effectors of the same side. Two observations 
may be cited in favor of the latter hypothesis : (i) There is a per- 
ceptibly lower degree of irritability during the periods of irregular- 
ity and asymmetry in response. My experiments are not exhaust- 
ive on this point, but they afford a considerable evidence to this 
effect, and none to the contrary. (2) The irritability of an embryo 
may vary perceptibly within a comparatively short period of time. 
This factor has not been definitely correlated with irregularity in 
response, but it may be the explanation of the occasional move- 
ment towards the side touched during the long period of predomi- 
nant regularity. Also the very rare irregular movement occurring 
before a period of asymmetry, as observed above, may have its 
