ANATOMY OF A 17.8 MM. HUMAN EMBRYO 37 



fusion of these two parts to be a V-shaped groove with the apex of 

 the V pointing caudad. In plate 1 , the apex and right hmb of this 

 groove, sulcus tenninalis, are represented, the apex marking the 

 place of origin of the median thyreoid gland. Laterally the tongue 

 is bounded by deep alveolo-lingual grooves which converge ceph- 

 alad so as to separate its tip from the subjacent mandible. 



According to Kallius ('01) the anterior anlage of the tongue is 

 derived chiefly from the dorsum of the ventral ends of the man- 

 dibular processes, the so-called lingual folds, the tuberculum 

 impar of His contributing only a small part thereto. Hammar, 

 however, believes ('01), that the tuberculum impar is a transitory 

 structure, and that the tip and body of the tongue are formed by a 

 considerable area of the floor of the oral (pharyngeal) cavity. In 

 regard to the development of the root of the tongue there is some 

 disagreement in that His derived it ('85) from the ventral ends 

 of the second and third visceral arches, while Born ('83) and 

 Hammar ('01) limit it to the second arch. 



Posterior to the root of the tongue, and fused with it, there 

 is a broad, bilobate elevation (Ep.) which represents the epi- 

 glottis. It is a derivative of the third visceral arch (Born '83, 

 and Hammar '01). 



The first pharyngeal pouch {Ph.P.l, plate 5), the cavity of 

 which ultimately, will form the tuba auditiva and cavum tym- 

 pani, is seen at this stage to be an extensive, lateral, pointed, 

 evagination of the pharyngeal wall, extending somewhat dorsally 

 toward the depression of the primary meatus acusticus externus. 

 It presents three surfaces, dorsal, cephalo-ventral, and caudo- 

 ventral. The dorsal surface, which cannot be seen in the draw- 

 ing, is triangular in outline; medially it passes over into the dorso- 

 lateral wall of the pharynx. Dorsal to it is the cochlear division 

 of the otocyst. The caudal boundary of this surface is the poste- 

 rior tympanal ridge. The cephalic boundary is marked by a 

 ridge overlying the tubo-tympanal sulcus of Moldenhauer ('77). 

 This ridge extends from the tip of the first pharyngeal pouch in a 

 cephalic and medial direction to the oral epithelium between the 

 hypophysis and the angle of the mouth. The cephalo-ventral 

 surface of the first pharyngeal pouch is concave. A dorso-ven- 



