254 G. E, Coghill 
basis in this variable irritabilityj at some point in the neuro-muscu- 
lar 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 stimu- 
lus, the reponse of a symmetrically developed, normal embryo of 
Diemyctylus torosus is regularly away from the side touched when 
the stimulation is applied to the fields of the n. trigeminus and n. 
vagus. 
b. Response to Stimulation of the Tail Bud. 
There is no marked regularity in the responses to touches on the 
tail bud. There is a slight general tendency in some specimens 
towards movement of the head toward the side touched, but no 
definite significance can yet be attributed to this tendency. It is 
clear, however, that specimens that are asymmetrical with reference 
to stimulation on the head are similarly asymmetrical with reference 
to stimulation of the tail bud, and that ordinarily the asymmetry 
with reference to the two points of stimulation extends over 
approximately the same period. 
One other fact concerning the reaction to stimulation on the tail 
bud is established beyond question by my experiments. The first 
response to such a stimulus in the very young embryo is a head 
movement, and as the embryo advances in age this movement still 
begins in the head region and progresses caudad. Ontogenetically, 
then, the most primitive conduction paths of the medulla spinalis 
are longitudinal and afferent, and the crossed paths are secondary, 
excepting possibly in the most cephalic part where the medulla 
spinalis may be involved in the crossed paths between the n. trigem- 
inus or n. vagus and the opposite musculature of the trunk. The 
two halves of the medulla spinalis, therefore, seem to be physio- 
logically distinct during this phase of development. This fact of 
development reveals from a new source the fundamental nature of 
the longitudinal divisions of the cerebro-spinal system, at least of 
the somatic components, as they have been conceived by Herrick,^ 
^The Cranial and First Spinal Nerves of Menidia, Archives of Neurology and 
Psychology^ voL ii, and The ’Journrl of Comparative Neurology^ vol. ix; also numer- 
ous later papers, mostly in this Journal. 
