DEVELOPMENT OF REFLEX MECHANISMS 73 
side and thus initiate a descending wave of muscular contraction 
which follows the first but in the opposite direction. This proc- 
ess is continued and repeated as long as the swimming movement 
persists, thus giving rise to the serpentine swimming reflex. 
This mechanism obviously is adapted to produce only one 
form of response to any sort of excitation applied to the trunk 
region of the embryo, viz., a swimming reaction, and the same 
neurones are involved throughout in both the exteroceptive and 
the proprioceptive factors in the reaction. Physiological experi- 
ments show that this in fact is the case. 
At a later period in development the transitory dorsal giant 
cells disappear and are replaced by typical spinal ganglion 
neurones which have meanwhile differentiated from the neural 
crest. And no doubt the adult Amblystoma, like the higher 
vertebrates, possesses distinct neurones within the spinal ganglia 
to serve the exteroceptive and the proprioceptive functions respec- 
tively involved on the afferent side of the swimming reflex. 
In older larvae the efferent side of the reflex circuit also shows 
an advance beyond that of the youngest swimming embryo, 
where the motor root fibers arise as collateral branches from the 
descending tract fibers. The details of the transformation 
have not been observed; but it is probable that in some of these 
neurones the descending tract fiber elongates and develops at 
the expense of the peripherally directed collaterals, while in others 
a collateral develops at the expense of the descending fiber, thus 
transforming the element into a typical ventral horn neurone. 
SPINAL REFLEXES IN THE HALF GROWN LARVAE 
The spinal cords of urodele larvae have been investigated 
histologically by several observers, the most important of these 
studies being those of van Gehuchten (’97) upon Salamandra. 
Most of his observations have been confirmed upon Amblystoma 
larvae and figure 7 presents a diagram, which elucidates some of 
these relutions. This mechanism is much more complex than that 
of the earliest swimming larva, though still very different from 
that known in the spinal cords of adult higher vertebrates. 
