The Reaction to Tactile Stimuli 
257 
I Now, so far as my observations go, the S reaction never appears 
until the embryo is capable of executing an extended general flexure, 
and rarely until it has actually executed a coiled reaction. Fur- 
thermore the S reaction is ordinarily first performed by a reversal 
of the head from an extended general flexure or a coiled reaction 
before the original flexure is completed in the caudal part of the 
i trunk. This reversed movement of the head, in early stage of the 
embryo, may simply progress caudal till it reverses completely 
the original flexure; but when the movement attains its typical 
form it is a relatively short, quick movement, and, when performed 
in series, it becomes the normal swimming movement. 
The occurrence of the S reaction in series has its origin, evidently, 
in a mode of response which appears very early in the course of 
development. It may be designated as the ‘Secondary reaction.’’ 
This secondary reaction is a movement that is made during the 
phase of relaxation from a direct lesponse to an external stimulus. 
It is caused, probably, by a rhythmic process in the motor cells, 
or, possibly, by stimuli from the proprio-ceptive field. It may be 
of greater or less extent than the original flexure. It may, for 
instance, advance a general flexure into a coiled reaction. It is a 
conspicuous feature in the behavior up to the time when the S 
reaction appears. 
Now, it is obvious that when the head is once reversed from a 
flexure into an S reaction, the secondary reaction would explain the 
second reversal, which is simply repetition of the initial movement. 
The successive reversals of the head may, then, be initiated as 
secondary reactions and the progression of the successive flexures 
caudad, in the form of S reactions, propels the animal forward. 
Locomotion, therefore, in the amphibian embryo is dependent 
upon the progression of the flexure cephalo-caudad, and the cepha- 
lo-caudal progression of the individual movement is further corre- 
lated with a similar progression in the ontogenetic development of 
the reaction. Furthermore, it is clear that this order of develop- 
ment of function is correlated with the order of structural develop- 
ment of the central nervous system, as illustrated, for instance, in 
the order of closure of the neural tube. These correlations natu- 
rally suggest, further, that the necessity of locomotion may have 
been an important phylogenetic factor in determining the order of 
development of the parts of the nervous system in vertebrates. 
Emphasis, properly, has been placed, by authorities generally. 
