DIFFERENTIATION OF AMPHIB I AN NERVOUS SYSTEM 385 



cells in the ventral region of the spinal cord, but it results in a 

 60 per cent, decrease in the number of sensory neurons in the 

 dorsal-root ganglia of that side.^ Unilateral removal of the 

 muscles (which of course contain proprioceptive receptor organs), 

 without injuring the skin overlying them, results in a 40 per cent, 

 decrease in the number of sensory neurons in the dorsal-root 

 ganglia of that side, again without affecting the number of motor 

 neurons in the ventral part of the spinal cord.^ Incidentally, it 

 may be observed that these results give interesting information as 

 to the proportion in which exteroceptive and proprioceptive 



Fig. 186 



I, Amblystoma embryo showing area of epidermis removed. 2, Two embryos 

 grafted together and lacking epidermis, and therefore sensory load, on their 

 inner sides. (From Detwiler, 3^07^;7z. Exp. Zool. xlv, 1926.) 



neurons occur in the dorsal-root ganglia, since the skin contains 

 exteroceptors and the muscles proprioceptors. 



All these experiments have an effect only on the number of 

 neurons in the dorsal-root ganglia, i.e. of sensory neurons, and the 

 effect has in all cases been due to an increase or decrease in the 

 sensory region from which the sensory cell-area (the ganglia) in 

 question receives impulses. On the other hand, the number of 

 motor neurons in the ventral region of the spinal cord, i.e. the multi- 

 plication of cells in the motor cell-area, is controlled by the number 

 of endings of axons of the descending tracts of the cord (tractus 

 bulbo-spinalis). Thus if the region of the first five segments of the 

 spinal cord is removed, and in its place an extra medulla oblongata 

 ^ Detwiler, 1926 a. ^ Detwiler, 1927 a. 



HEE 25 



