248 ALIOS JOHNSON AND LILIAN SHELDON. 



of the 7Lh nerve. The ganglion retreats further from the 

 surface of the body^ remaining attached by the dorsal nerve to 

 the sense organ (fig. 34)^ and the ventral portion of the main 

 trunk divides into two branches, the post-branchial (fig. 35) 

 behind the second gill-cleft, and the prae-branchial (fig. 33) in 

 front of it. 



The Vagus arises from the brain in the same manner as 

 the other cranial nerves, but we have not traced its further 

 development. 



Development of Nerves in the Frog. 



We have made a few observations on the development of 

 the nerves in the Frog in some series of sections cut by Mr. 

 Durham, and very kindly lent to us. Our observations, as far 

 as they extend, confirm in every respect wliat we have described 

 in the Newt. A neural ridge is formed on the dorsal surface 

 of the medullary canal after it has separated from the epiblast, 

 as shown in fig. 30, representing part of a transverse section 

 through the hind region of the trunk of an embryo. In this 

 embryo the neural ridge extended through the trunk, but was 

 less distinct in the head, where the nerves had begun to form 

 as outgrowths from it. Fig. 29 shows the origin of the facio- 

 auditory nerve. Its small size shows that it must be at a very 

 early stage. It is growing on each side from the dorsal surface 

 ot the brain towards the auditory vesicle, which is beginning 

 to develope from the inner layer of the epiblast. It seems to 

 us that the whole appearance is inconsistent with the view 

 that the nerve has split ofi' from the epiblast, as Mr. Spencer 

 asserts (21). 



Historical and Critical. 



Our observations are, on the whole, consistent with the 

 account of the derivation of nerves first put forward by Pro- 

 fessor Balfour in 1876 (1), afterwards confirmed by Professor 

 Marshall in other types, and since generally accepted. They 

 do not lend any support to the peculiar view of His, as to the 

 presence of a " Zwischenstrang^' (11)- 



