CoGHiLL, Cranial Nerves of Auiblystoma. 267 



tube in Rana and Bufo has been traced b}- Spemann ('98) and 

 (Fox) ('01) in its ontogenesis from the embryonic spiracular 

 cleft. This nerve, therefore, is post-spiracular. On the other 

 hand, the r. alveolaris in larvae of Amblystoma, in reaching 

 the lower jaw, passes anteriorly (cephalad) of the deep 

 pharyngeal evagination which represents the embryonic spirac- 

 ular cleft. Furthermore, having reached the lower jaw, the r. 

 mandibularis internus still lies morphologically behind the posi- 

 tion of the r. alveolaris. In young larvae of Amblystoma the 

 r. alveolaris lies dorsally of the m. mylo-hyoideus, a position 

 which the nerve always holds in Proteus and Menobranchus 

 (Druner). In older larvae and in the adult of most Urodela 

 the nerve becomes enveloped in a canal by the ossification of 

 the mandible and its relative position to the muscles may 

 become somewhat changed here, but when it emerges from the 

 canal it emerges dorsally of the musculature. In Anura, on 

 the contrary, as Gaupp ('97) clearly states, the r. mandibularis 

 internus lies ventrally of the m. mylo-hyoideus and sends its 

 fibers through this muscle to innervate the oral epithelium. 

 This is essentially the relation of the r. mandibularis internus of 

 fishes, also. 



Still other important differences between these amphibian 

 nerves should be emphasized. In Amblystoma the r. alveo- 

 laris receives fibers from the glossopharyngeus and anastomosis 

 with the trigeminus. These relations do not occur universally 

 in Urodela, but the latter anastomosis is frequent and perhaps 

 universal. In none of these features does the r. alveolaris 

 agree with the r. mandibularis internus. 



In spite of these striking differences, however, the nerves 

 in question have two important features in common, area of 

 distribution and central termination. These are two of the 

 criteria of homology in nerves and they must be kept in mind 

 in considering the third criterion, the relation of the nerves to 

 other important structures. An explanation, therefore, which 

 is offered for the differences between the nerves must explain 

 also their features of perfect resemblance. 



Such an explanation as I have indicated is found by a com- 



