284 G. E. COGHILL 



that are not readily excited to motor response are much more 

 likely to move towards the side touched than are embryos that 

 are easily excited to movement. 



The conclusion of the last paragraph raises a question which 

 has been mentioned in an earlier paper ('09, p. 252) as to whether 

 movement away from the side touched is a fundamentally pri- 

 mary thing in behavior or a secondary thing, derived by a process 

 of selection out of a diffuse or irregular type of response. From 

 the conclusion reached concerning the structural basis for prompt 

 and certain response to stimulation of the trigeminal field, it 

 might be inferred that this problem also hangs directly and 

 solely upon the degree of development of the peripheral neurones 

 in their relation to the motor centers. In other words, if a 

 stimulus must pass through such a descending trigeminal tract 

 as is shown in figure 22, representing the early flexure stage, 

 there is a larger element of chance or accident in the direction of 

 the response than there would be if the stimulus traveled into 

 the immediate vicinity of the motor nuclei through the peripheral 

 neurones. This would be an attractive hypothesis, but, to my 

 mind, it is only of secondary consideration. When, for instance, 

 there is a high degree of responsiveness (a low threshold of stimu- 

 lation for the entire reflex arc concerned) a stimulus such as is 

 employed experimentally would act in a much more localized or 

 restricted area than it would when there is a low degree of 

 responsiveness. Under the latter condition the stimulus would 

 probably be transmitted to relatively distant receptors, possibly 

 even to the opposite side of the embryo and, finding here access 

 to an arc of lower threshold, it would excite a response that 

 apparently violates the regular law of behavior. This expla- 

 nation gains credence from Hooker's experiments ('11) which 

 show that amphibian embryos of the ages under consideration 

 are very sensitive to stretching of the skin by pressure upon 

 relatively distant points. In fact his experiments show con- 

 clusively that in order that a tactile stimulus act locally in any 

 appreciable degree in these embryos there must be a very low 

 threshold of stimulation for the reflex arc as a whole. Undue 

 resistance at any point in the arc would certainly allow the 



