4i6 Journal of Comparative Neurology and Psychology. 



of the nerves arc essentially the same as described for the sec- 

 ond embryo. In addition to the inferior dental and linf^ual 

 branches of the submaxillary, the mylohyoid and masseter 

 branches are clearly defined. The lingual occupies a position 

 along the inner side of Meckel's cartilage and can be traced 

 forward into the lateral region of the tongue. Of the facial 

 nerve, also, the supra-maxillary and auriculo-temporal are easily 

 traced in their usual positions. The chorda tympani is easily 

 traced from its point of origin from the facial trunk. It passes 

 behind Meckel's cartilage, takes the same general direction 

 that it does in the earlier embryo, and joins the lingual nerve 

 near the point of separation of the latter from the inferior den- 

 tal. But the relation of the chorda tympani to the spiracular 

 cleft is distinctly different from that found in the earlier embryo. 

 In its course in front of the hyoid cartilage and behind the 

 proximal end of Meckel's cartilage, it passes ova- the extreme 

 lateral end of the spiracular cleft and close to its dorsal edge. 

 It remains a question whether this lateral end of the spiracular 

 cleft is the primary end of the cleft or a secondary evagination 

 from it. It might be the latter, since it is generally accepted 

 that the closed end of the ceft by evagination outward towards 

 the external auditory meatus and upward around the chorda 

 tympani and auditory ossicles, forms the tympanic cavity of the 

 adult. 



We find, then, at this stage of development that the chorda 

 tympani no longer lies underneath the spiracular cleft but that 

 it passes over the closed end of the cleft, or over the funda- 

 ment of the tympanum. This is the morphological position 

 the nerve holds in the adult. 



Conclusions. 



The results of this study of the embryological development 

 of the chorda tympani in Microtns lead to the following con- 

 clusions : 



I. In the earlier stages of development the chorda tym- 

 pani passes behind and underneath the spiracular cleft. 2. In 



