128 THE FROG. 



the growth of processes from the cells of the nei'ves into the 

 substance of the brain, or spinal cord. 



Up to this point th e develo Bment of the cranial and t he 

 s pinal nerves is practically the same ; the cranial nerves appe ar 

 at an earlier stage in the formation of the neural tube than d o 

 the spinal nerves, and a re from the first of much larger s ize 

 t han these latt er, but the history of the earlyj tagfis in e s se ntially 

 the same in the two cases. 



Che 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 

 slo wly down between the myotomes and the s ides of _t he sp inal 

 cord. The permanent attach ment to the side of the cor d is 

 acquired in the manner described above, "by growth of nerve 

 fibres from the nerve rudim ent, or ganglion, into the cor d" The 

 g anglion itself enlarge s, and the nerve fibres continue their 

 course beyond it to form the tru nk of the dor sal or sensory root 

 o fThe spinal nerv eT 



T he ventral or moto r ro ot arises quite ind ependently : the 

 details of its development have not been determined so accu- 

 rately in the frog as in other animals, bu t each ventral r oot 

 a ppears to arise as a number of outg rowths from th e lower part 

 of the side of the spinal cord , whi ch from the first occupy their 

 per manent positions in regard to the cor d, a nd which very ear ly 

 be come conne cte d distally with the muscles of the body . 



Th e dorsal and ventral roots of each nerve lie close alo ng- 

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

 n ective-tissue sheath to for m the trunk of the sip in al nerve, from 

 w hich branch es soon arise supplying the various p arts to which 

 the nerve is"3istributed in the adult (cf. Figs. 87 and 88). 



The cranial nerves. The development of the cranial nerves 

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

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

 factory condition.- Th e nerves which are undoubtedly deriv ed 

 f rom 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 



