The Reaction to Tactile Stimuli 
243 
to the margin of the dorsal or ventral caudal fin and a movement 
of the head only results, as regularly occurs in certain phases of 
development, it is absolutely impossible for such a reaction to be 
given in response to pressure either upon the acting muscles or upon 
the central nervous system. As reactions of this sort occur here 
and there throughout nearly every one of my experiments, it seems 
to me certain that the stimulus employed vcas, with possible rare 
exceptions, purely tactile, and that, so far as the mode of stimula- 
tion is concerned, my conclusions are valid. 
Ordinarily the stimulus was applied to the upper side of the 
specimen as it lay on its side on the bottom of the dish. Frequently 
however, it was applied to the under side of the specimen from 
beneath, in order to determine whether contact with the dish had 
any influence on the mode of reaction, but it was impossible to 
detect any factor of this kind in the responses. Some embryos, 
also, were suspended in an upright position and tested for the same 
purpose, and with the same result. 
An individual record in detail was kept of each embryo from the 
time it was removed from the egg membranes till the end of the 
experiment. In the record of each trial, or application of the 
stimulus, the following factors were noted particularly: the region 
and side touched, the form of the response and the time of the trial. 
Tabulated schemes for rapid recording were tried in my first experi- 
ments of 1906, but it soon became apparent that such forms could 
not be adhered to, for they were necessarily based upon presump- 
tions of some sort and were, therefore, a hindrance rather than a 
help to alert observation. These methods were wholly abandoned 
and have no part in the records from which this paper is written. 
REACTION TO TACTILE STIMULI. 
a. Response to Stimulation on the Head. 
According to their reaction to a touch on the side of the head, 
in the region innervated by n. trigeminus or n. vagus, embryos of 
Diemyctylus torosus may be grouped according to three types, as 
follows : 
Type I. Embryos which from the beginning and during a con- 
siderable period, respond regularly or almost regularly with a 
movement of the head directed away from the side touched. 
