414 Journal of Coinparativc Neurology and Psychology. 



as typical visceral arches. The formation of the mandibular 

 and hyoid cartilages has not begun, and these regions are filled 

 with primitive mesenchyme cells. 



T/ie Spiraailar Cleft} — In the dorsal part of the external 

 groove between the mandibular and hyoid arches a small pit is 

 found. From the apex of this pit the lumen of the spiracular 

 cleft passes inwards and cephalad, and opens into the pharynx. 

 Beginning in its most external part the lumen is very narrow for 

 a distance of about .07 mm., then it broadens out into a flat 

 cavity with its shortest diameter, as seen in the sagittal sec- 

 tion, in a dorso-ventral direction. In the extreme lateral region 

 of the cleft its epithelial walls approach each other in such a 

 manner that the lumen is reduced to a small circular canai, un- 

 til, finally, for an extremely short distance they come into close 

 contact with each other, so that the lumen seems to be oblit- 

 literated. The epithelium of the cleft, hoivever, is pcrfectlv con- 

 timions with the epithelium of the skin. 



This relation of the visceral arches and the pharyngeal 

 cavity to this cleft, and the continuity of the inner and outer 

 epithelial plates are essential characteristics which establish its 

 homology with the spiracular cleft of fishes. 



TJie Cho7-da Tympani and Related Nerves. — The Gasscrian, 

 geniculate and auditory ganglia and their roots are clearly de- 

 fined. The rr. ophthalmicus, maxillaris superior and maxillaris 

 inferior are easily identified in their usual relations. Near its 

 ganglion the r. maxillaris inferior gives off the buccal nerve, 

 and, passing into the mandibular arch, divides to form the in- 

 ferior dental and the lingual nerves. The latter can be traced 

 into the base of the tongue. 



From the geniculate ganglion the facial nerve passes out- 

 ward and slightly caudad for some distance. Just back of the 

 spiracular cleft it makes a slight turn ventrad. At this angle it 

 gives off the chorda tympani from its anterior border. The 



' 'Vhii iurm spiracular ch'/t \s used here as IIertwk; uses llic term sulcus 

 tubo iyinpunicus. 'J'liis usage is justified by the relations as tliey are described 

 farther on in this paper. 



