ii8 Journal of Comparative Neurology. 



developed than in the thoracic. Just why the cervical and lum- 

 bar roots should be so far ahead of the thoracic root in this re- 

 spect is not at the moment readily explained. 



The increase in the total number of fibers in the dorsal nerve 

 roots is only to be explained by the outgrowth of nerve fibers 

 from the spinal ganglion. It follows from this explanation that 

 in the immature rat we should expect to find in a given dorsal 

 root a larger number of the fibers near the ganglion than at the 

 entrance of the root into the spinal cord. On this point, 

 Hardesty ('99) obtained striking results from counting the 

 fibers in the dorsal roots in the frog. He says " The number 

 of fibers in the dorsal roots decreases from the spinal ganglion 

 towards the spinal cord." This observation has been fully con- 

 firmed in his most recent paper ('00) on the same subject. On 

 the other hand, Dale ('00). who counted the number of coccy- 

 geal dorsal root fibers in the adult cat at different levels was un- 

 able to find such numerical differences in the two levels, one 

 near the cord and the other near the ganglion. He says, "The 

 number of the fibers close to the ganglion is the same as the . 

 number of fibers several millimeters from it, both proximally 

 and distally, i. e., none of the meduUated fibers given off by 

 the ganglion cells end in the nerve roots close to the ganglion." 

 This observation by Dale has been already explained by Hard- 

 esty as follows : "That Dale's result does not agree with the 

 results previously obtained and here extended is probably due 

 to the fact that the growth of the nervous system of the frog is 

 much slower than that of the mammal. The cat has a fixed 

 period of growth, while the frog, if it does not grow as long as 

 it lives, at least cannot be said not to do so-" A similar ex- 

 planation has been givin by the present writer in his previous 

 paper ('01, I) on the finer structure of the spinal ganglion cells 

 in the white rat. 



