68 C. J. HERRICK AND G. E. COGHILL 
The axones of the neurones last mentioned give off collaterals 
which form ventral root fibers and thus innervate the muscles 
(fig. 3). 
In the youngest stages which are capable of responding at all 
to external stimuli, commissural fibers cross at one region only, 
viz., the upper end of the spinal cord and lower end of the medulla 
oblongata. In older stages, this ventral commissure is extended 
Ventral commissure -__ 
Sensory fract.__ 
Motor fract..__ 
Motor roof-.._- 
Sensory root... _7_ 
Fig. 3 Diagram of a dorsal view of the upper end of the spinal cord of larval 
Amblystoma of the same.age as in figure 2, to illustrate the relations of the giant, 
cells, commissural cells and motor cells and the mechanism of the swimming 
reflex. Based on the researches of Coghill (713, 714). 
both forward and backward from this level until finally it. extends 
throughout the entire central nervous system: below the optic 
chiasma, and other types of connection between the peripheral 
sensory and motor neurones are effected, so that local crossed 
and uncrossed reflexes are possible in every part of the central 
nervous system. 
