128 THE FROG. 



the growth of processes from the cells of the nerves into the 

 substance of the brain, or spinal cord. 



Up to this point the development of the cranial and the 

 spinal nerves is practically the same ; the cranial nerves appear 

 at an earlier stage in the formation of the neural tube than do 

 the spinal nerves, and are from the first of much larger size 

 than these latter, but the history of the early stages is essentially 

 the same in the two cases. 



The spinal nerves. After reaching the neural crest stage, 

 the development of the spinal nerves proceeds for a time very 

 slowly. The nerve rudiments, after a rather long pause, grow 

 slowly down between the myotomes and the sides of the spinal 

 cord. The permanent attachment to the side of the cord is 

 acquired in the manner described above, by growth of nerve 

 fibres from the nerve rudiment, or ganglion, into the cord. The 

 ganglion itself enlarges, and the nerve fibres continue their 

 course beyond it to form the trunk of the dorsal or sensory root 

 of the spinal nerve. 



The ventral or motor root arises quite independently : the 

 details of its development have not been determined so accur- 

 rately in the frog as in other animals, but each ventral root 

 appears to arise as a number of outgrowths from the lower part 

 of the side of the spinal cord, which from the first occupy their 

 permanent positions in regard to the cord, and which very early 

 become connected distally with the muscles of the body. 



The dorsal and ventral roots of each nerve lie close along- 

 side each other, and become bound together by a common con- 

 nective-tissue sheath to form the trunk of the spinal nerve, from 

 which branches soon arise supplying the various parts to which 

 the nerve is distributed in the adult (cf. Figs. 87 and 88). 



The cranial nerves. The development of the cranial nerves 

 of the frog has not been very thoroughly studied ; and there are 

 several points on which our knowledge is still in an unsatis- 

 factory condition. The nerves which are undoubtedly derived 

 from the neural ridges are the trigeminal, the facial and auditory, 

 and the sensory branches of the glosso-pharyngeal and pneumo- 

 gastric ; i.e. the fifth, seventh, eighth, ninth, and tenth cranial 

 nerves according to the ordinary nomenclature. The olfactory 



