Em MEL, Chorda Tympani in Microius. 415 



chorda tympani then passes directly forward beneath the spirac- 

 Lilar cleft and close to its ventral edge. It soon turns inward 

 and passes a considerable distance nearly parallel with the an- 

 terior wall of the cleft. Near the rudiment of the tongue it 

 meets and fuses with the lingual brach of the trigeminus. 



At this stage of the embryonic development of Microtiis, 

 therefore, the primitive continuity of the epithelium of the 

 spiracular cleft and the skin still persists, and the chorda tym- 

 pani passes behind and underneath the cleft and unites in a 

 typical manner with the lingual nerve. 



Third Embryo. 



In the oldest of the three embryos, from which projections 

 were made as from the second embryo, the pinna has begun to 

 form, the mandibular and hyoid arches no longer appear as vis- 

 cerial arches and have assumed in a general way the adult con- 

 ditions. The skeletal regions are still for the most part filled 

 with mesenchyme cells, but the fundaments of Meckel's car- 

 tilage and of the hyoid cartilage are distinguishable. 



The Spiracular Cleft and External Aziditory Meatus. — 

 The comparatively large orifice of the external auditory meatus 

 is bordered by the ridges or fundaments of the pinna. The 

 meatus soon narrows into a flattened cavity with its shortest 

 diameter lying in the dorso-ventral plane, as seen in sagittal sec- 

 tion. Its course is in a cephalo-ventral direction. It terminates 

 as a blind tube. In a plane about .075 mm. outward from the 

 blind end of the external auditory meatus, and dorsal of the 

 meatus at a distance about equal to the greatest diameter of the 

 meatus at this region, lies the blind end of the cleft which in 

 the second embryo was identified as the spiracular. This cleft 

 is also a flattened cavity with its longest diameter, as seen in 

 sagittal section, lying in nearly the horizontal plane. It passes 

 inward for a short distance, then takes a cephalo-ventral direc- 

 tion and opens into the pharynx. It is an important fact that 

 in this embryo there is no continuity between the epithelium 

 of the spiracular cleft and the skin. 



The Chorda Tympani. — The ganglia and the main trunks 



