96 Journal of Comparative Neurology and Psychology. 



the same musculature is through the asceudiug axoues of the afferent 

 nerves. This factor will be best considered in connection with the 

 account of reaction to touch on the tail bud. 



The most difficult phase of the problem to deal with bj 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: (1) There is a perceptibly lower degree of irritability 

 during the periods of irregularity and asymmetry in response. My 

 experiments are not exhaustive on this point, but they afford a con- 

 siderable evidence to this effect, and none to the contrary. (2) The 

 irritability of an embryo may vary perceptibly within a compara- 

 tively short period of time. This factor has not been definitely 

 correlated with irregularity in response, but it may be the explanation 

 of the occasional movement towards the side touched during the long 

 period of predominant regularity. Also the very rare irregular 

 movement occurring before a period of asymmetry, as observed above, 

 may have its basis in this variable irritability at some point in the 

 neuro-muscular system. 



In some such manner as indicated above my experiments permit 

 of a provisional hypothesis to explain the occurrence of the early 

 periods of asymmetry and irregularity in response of some embryos 

 and the occasional movement towards the side touched, and warrant 

 the conclusion that, for a period of about 48 hours, or more, following 

 the first movements in response to a tactile stimulus, the response of 

 a symmetrically developed, normal embryo of Diemyctylus torosus 



