284 C. D. SNYDER. 



objected that results obtained from an animal of such pronounced 

 metameric structure as that of the earthworm cannot be applied 

 in the case of vertebrates ; that if " the nerve-cord of the earth- 

 worm is an apparatus by means of which reflexes may be trans- 

 ferred from one segment to the other '" it does not follow that 

 the nerve-cord of a vertebrate in its activity can be reduced to 

 elements equally simple. 



In uninjured land animals of the bilateral types progressive 

 locomotion is usually initiated by forward or lateral movements 

 of the anterior part of the body. This action produces an effect 

 on the passive posterior part of the body, especially in the skin, 

 where sensory stimuli may be set up. These sensory stimuli 

 may be aroused in two ways. First, by tugging, the skin is 

 subjected to stretching stimuli ; second, by dragging even the 

 slightest distance, the ventral surface of the body, and therefore 

 of the skin of creeping animals, is subjected to a series of rapid, 

 successive impacts against the inequalities of the ground-surface. 

 In other words, by dragging the skin is subjected to friction stimuli 

 in addition to stretching stimuli. 



Now since the skin is always subjected to more or less such 

 treatment in the initiation of locomotor movements, it is reason- 

 able to suppose that a characteristic combination of sensory stimuli 

 is thereby produced which may be able to set up motor stimuli 

 in centers of corresponding levels. One may suppose moreover 

 that these motor stimuli do not result in vague, purposeless re- 

 flexes, but rather that they result in definite, unvarying coordina- 

 tions ; that they are operative and effective without intervening 

 stimuli from other regions of the nerve-cord. 



In other words, a definite combination of sensory stimuli in one 

 region, occasioned by a definite motor activity in another region, 

 may be a sufficient signal for locomotor activity in the region re- 

 ceiving sucli sensory stimuli. 



4. At the suggestion of Dr. Loeb, for whose constant assis- 

 tance and kindly interest in my work I wish here to express my 

 best thanks, I began some experiments during February of this 

 year to test the truth of the ideas set forth in the foregoing para- 

 graphs. 



! Freidlander,' loc. cit., p. 206. 



