CoGHiLL, The Reaction to Tactile Stimuli. 95 



and numerous other embryos is not based upon relative age but upon 

 the relative development, and probably the variable physiological 

 condition, of the various constituent elements of the neuro-muscular 

 system. When a period of asymmetry occurs, it appears before the 

 period of irregularity or regularity, and never follows either of the 

 latter, excepting in rare cases when one or two movements right at 

 the beginning of the experiment are at variance with the asymmetry 

 (Figs. 3, 4, 5, 6). The asymmetry clearly influences the irregular 

 reaction in some cases, so that the movements towards the side 

 touched appear to bo determined by a partial persistence of asymmetry 

 (Fig. 5). But this is not always the case. The period of regularity 

 persists, ordinarily, till near the time of swimming. The actual 

 length of the period varies greatly in different specimens, but a 

 comparative study of numerous specimens convinces me that the regu- 

 larity in response is purest for a period of about 48 hours. 



The structural basis for a regular asymmetry in response must be in 

 the ascendency of the effector system of one side over that of the other, 

 rather than in structural difference in the receptor systems of the two 

 sides. Two facts particularly support this interpretation: (1) All 

 spontaneous movements (somatic) that have been observed in embryos 

 which conform to a given asymmetry are in accordance with the asym- 

 metry in each case, towards the right in dextrally asymmetrical speci- 

 mens and towards the left in sinistrally asymmetrical specimens. (2) 

 In any given asymmetrical embryo the asymmetry is the same with 

 reference to stimulation on the tail bud as it is with reference to 

 stimulation on the head, and specimens that are asymmetrical in 

 one respect are so also in the other. 



The stnictural basis for a regular movement of the head away 

 from the side touched must obviously lie in the ascendency of the 

 descending tracts which decussate in the cephalic part of the central 

 nervous system over the uncrossed long tracts which descend into 

 the cord. In comparing this condition with the response to stimula- 

 tion on the tail bud, it should be remembered that the path from n. 

 trigeminus or n. vagus to the opposite musculature of the cephalic 

 part of the trunk is through the descending axones of these nerves 

 within the central system, while the path from the caudal nerves to 



