DEVELOPMENT OF REFLEX MECHANISMS 67 
THE EARLIEST SWIMMING REFLEX 
It has been shown (Coghill, ’09, 713, 714) that in Amblystoma 
the earliest somatic movements in response to external stimu- 
lation are possible only after the completion of a definite anatomi- 
cal circuit in the nervous system (though the muscles may be 
excited to contraction by the direct application of an electrical 
stimulus prior to this age). The first movement which can be 
evoked by light tactile stimulation of the skin is a simple avoid- 
ing reaction—at first turning the head end of the body away 
from the side stimulated, then a convulsive ‘coil reaction,’ then 
an ‘S-reaction,’ which is immediately followed by a true swim- 
ming reflex. 
Je 
Sensory’ 
tract 
Myotome 
Fig. 2 Diagrammatic cross section through the body of an early swimming 
stage of larval Amblystoma, to illustrate the connections of the transitory giant 
cellsof Rohon-Beard. Based on the researches of Coghill (’14). 
During these periods the nervous system has the structure 
shown diagrammatically in figures 2 and 3, the peripheral sen- 
sory neurones lying within the spinal cord (the transitory dorsal 
giant cells of Rohon-Beard) and sending dendritic processes 
outward to both the skin and the myotomes. The axones of 
these cells ascend in a dorso-lateral sensory tract, whence they 
are discharged into a series of commissural cells, whose axones 
cross to the opposite side in the ventral commissure, after which 
they effect connection with cells of the descending motor tract. 
