The Reaction to Tactile Stimuli 
255 
Johnston^ and others on purely morphological and physiological 
grounds. It also suggests that in their direct connection with the 
cephalic part of the nervous system the special cutaneous systems 
of fishes and amphibians accord essentially with the primary plan 
of the general cutaneous system. 
It would be a difficult thing ordinarily to demonstrate that the 
receptive fields and afferent conductors become functional in an 
embryo before the effectors do, for through the effectors alone is the 
functioning of the receptor and conductor demonstrable. But if 
the skin of a given somite in the tail bud of an amphibian embryo 
of suitable age be touched there will be no perceptible response in 
the effectors of that segment, while response will occur in the older 
somites farther cephalad. Into this given caudal somite, then, 
impulses are pouring from the external world through the receptors 
and conductors before the effectors of that segment are capable of 
making any perceptible response whatever. If this is true of the 
more caudal somites, it may be assumed to be true of the head 
segments also, and the embryo may be regarded as existing under 
a storm of impulses of the receptive system for a considerable 
period before it has the ability to give expression through its 
effectors. Flow widely this order of development of the receptor 
and effector may be applicable, as a law, and what its significance 
may be are questions of interest. It is possible that the summation 
of subliminal stimuli in neuro-muscular reflexes rests upon this as 
a fundamental principle of functional development. It is possible, 
also, that Kappers^ might correlate this precocity of the afferent 
system with his theory of neurobiotaxis, in which he assumes that 
the afferent conductors have influence over the effector centers to 
cause them to migrate, phylogenetically at least, in the direction of 
the maximal amount of stimulation. 
^The Brain of Acipenser. Z06I. Jahrb., 1901; The Nervous System of Verte- 
brates, Philadelphia, 1906; and other papers in this Journal. 
® Phylogenetische Verlagerungen der motorishen Oblongatakerne, ihre Ursache 
und Bedeutung. Neurol. CentralbL, no. 18, 1907. 
Weitere Mitteilungen beziiglich der phylogenetischen Verlagerung der motori- 
schen Hirnnervenkerne. Der Bau des autonomen Systemes, Folia Neuro-Bio- 
logtca, B., Nr. 2, January, 1908. 
Weitere Mittheilungen iiber Neurobiotaxis. Folia Neuro-Biologica, B. i, Nr. 
4, 1908. 
The Structure of the Autonomic Nervous System Compared with its Functional 
Activity. Journal of Physiology, vol. xxxvii, no. 2, 1908. 
