242 
G. E. Coghill 
development of a “simple reflex” into a form of ackno’wledged in- 
stinctive behavior, this link in the development of behavior would 
seem to appear in the development of the swimming movement as 
described in the following pages. 
In view of this bearing of the experiments upon the subject of 
animal behavior certain results of the experimental part of my in- 
vestigations are here made known before the anatomical phase of 
the work has been completed. 
METHODS. 
The embryos were removed from the egg membranes at various 
stages in development, ordinarily before they showed any sign of 
irritability to tactile stimuli. They were then placed in shallow 
Petri dishes, a single specimen in a dish, and tested from time to 
time for reactions. Usually an experiment continued until the 
animal began to swim. 
The stimulus employed was a touch with the end of a rather fine 
human hair, mounted in such a way as to render the touch very 
gentle. The extreme sensitiveness of some very young embryos is 
remarkable. Even the touch of a fine piece of lint will at times 
evoke a vigorous response, as if it were a violent irritant. 
Without critical consideration the tactile nature of this mode of 
stimulation might be held in doubt. The touch of a hair such as 
was used in these investigations might easily cause a considerable 
pressure, so that there might be a question whether the responses 
were to a strictly tactile stimulus or to a mechanical stimulus upon 
the muscles or central nervous system. Indeed, in the very early 
phase of development, when the irritability was for some reason 
unusually low, some of the reactions, I believe, may have been to 
direct pressure upon the muscles or central nervous system. But 
such instances, if they occurred at all, in these investigations, were, 
I believe, relatively rare. For instance, when the stimulus is 
applied to the under side of the head as the animal lies on its side, 
and the response is a movement of the head away from the side 
touched, it is inconceivable that this response is to a direct pressure 
upon the muscles effecting the movement, and it seems altogether 
improbable that such a stimulus could be brought to bear upon the 
central nervous system directly in such a manner as to give rise to 
a constant form of response. Or, in case the stimulus is applied 
