No. I.] DESMOGNATHUS FUSCA. 243 



XII. The hypoglossus is included under the cranial nerves 

 with some misgiving. The consensus of opinion seems to be 

 against it in the Amphibia. In the Desmognathus a small 

 ventral nerve arises some distance caudal to the vagus outside 

 the cranial cavity and is possibly nothing more than the ventral 

 root of the first pair of the spinal nerves. 



Rolleston (45) states that there is no spinal accessory nerve 

 in Amphibia and that the area supplied by the hypoglossal is 

 supplied by the first spinal nerve in the majority. 



General Morphology. 



It is generally held by working neurologists that the adult 

 vertebrate brain shows evidence of differentiation into seg- 

 ments not necessarily correlated numerically with those found 

 in the embryo. 



In forms above the lancelet, five "definitive" encephalic 

 segments are commonly recognized : the prosencephal (includ- 

 ing the olfactory lobes), the diencephal, the mesencephal, epen- 

 cephal, and metencephal. Whether the olfactory portion 

 deserves setting apart as a separate segment under the name 

 of rhinencephal is a matter still under discussion. It is a 

 question for embryology and comparative neurology to deter- 

 mine. Wilder (58) has called attention to the potential triplicity 

 of these five segments which, although primarily mesal and 

 simple, present at some time and in some vertebrate, ontogenet- 

 ically or phylogenetically a threefold or tripartite condition. 

 In the prosencephal there is the mesal aula with its lateral 

 extensions, the paracoeles or lateral ventricles. The suggestion 

 is offered by him that a portion of the mesal aula may be con- 

 sidered as belonging to the rhinocoeles, thus bringing the 

 rhinencephal in line with the other segments. In the dien- 

 cephal, the diacoele and its optic recesses — remnants of the 

 optic vesicles ; in the mesencephal, notably in birds, the aque- 

 duct with its lateral extensions into the gemina ; for the epen- 

 cephal Wilder gives the epicoele for the mesal part and the 

 lateral recesses or parepicoeles for the lateral ; while the meten- 

 cephal, except in the Torpedo, Catostomus, and some other 



