THE NERVOUS SYSTEM OF AMPHIBIA 283 



ditioii, then, the significance of the rapid growth of the descend- 

 ing trigeminal tract between the coiled-reaction and early swim- 

 ming stages, for there is very close correlation between the 

 development of the power of locomotion and the introduction 

 of the most anterior sensory field of the organism into direct 

 connection with the motor centers so that it becomes effective 

 in determining the direction of locomotion. 



The significance of this correlation in the growth of the motor 

 and sensory systems finds a striking demonstration in the reac- 

 tions of embryos of advanced swimming stage which have been 

 transected in the upper portion of the trunk (Paper I, p. 200). 

 This operation cuts out both the exteroceptive and proprioceptive 

 stimuU from the greater portion of the trunk and leaves the 

 motor centers more exclusively to the influence of the cutaneous 

 field of the head. In head pieces of this kind, which have just 

 enough of the trunk musculature attached to give unequivocal 

 responses, the movements are almost uni\'ersally away from the 

 side touched. It is obvious, therefore, that this avoiding form 

 of response to stimuli about the anterior end of the embryo 

 is a basic thing in the orientation of the swimming animal to its 

 environment; and, judged by the anatomical results of this 

 paper, its perfection and eflSciency depend upon the growth of 

 the trigeminal root fibers into proximity with the motor centers. 



In the above experiments upon the relative irritability of 

 different areas it was observed that the regions untlcr consider- 

 ation differed materially with reference to the nature of the 

 response they evoke. In the first set, three of the five responses 

 to stimulation of the trigeminal area were away from the side 

 touched, whereas 62 of the 66 responses to stimulation of the 

 vagus field were away from the side touched, as were all of the 

 72 responses to stimulation of the spinal field. In the second 

 experiment mentioned the embryos were considerably older and 

 all of the 94 responses were away from the side touched excepting 

 two of the twelve responses to stimulation of the trigeminal area. 

 These data are in accord with my general experience with these 

 embryos, namely, that with increased responsiveness there is 

 increased purity of type in response. In other words, embryos 



