ALIMENTARY TRACT AND ITS APPENDAGES 305 



ing a stage in tooth formation, and rejected the interpretation. 

 Rose, however, has found the same ridge still better developed 

 in embryos of the tern and ostrich, and identifies it v^ry posi- 

 tively with the tooth-ridge or first step in the formation of the 

 enamel organ of other vertebrates. It seems probable that this 

 is the case, and that in this ridge we have the very last stage 

 of the disappearance of teeth. 



The Tongue. The tongue develops from two primordia in 

 the floor of the embryonic pharynx, one situated in front of, and 

 the other behind the thyroid diverticulum. The former, or 

 tuberculum impar, becomes manifest on the fourth day as a 

 slight rounded swelling situated between the lower ends of the 

 first and second visceral arches. The swelling is bounded behind 

 by a groove that has the ductus thyreoglossus for its center, and 

 in front by a shallow groove, that represents the frenulum, on 

 the posterior margin of the mandibular arches. The second 

 primordium, or pars copularis, arises just behind the thyroid 

 and includes the lower ends of the second visceral arches, a small 

 part of the lower ends of the third, and the region between these 

 arches. According to Kallius the tuberculum impar forms only 

 the center of the fore part of the tongue, and the lateral parts, 

 arise from two folds that form right and left of it (lateral tongue- 

 folds). The tuberculum impar thus expanded and the pars copu- 

 laris constitute two very distinct components in the development 

 of the tongue. 



Soon after the closure of the thyroid duct the two tongue 

 components become confluent, but the zone of junction remains 

 visible for a long time as a groove (cf. Fig. 148). Moreover 

 the epithelium of the forward component soon becomes thick- 

 ened and stratified, while in the pars copularis the epithelium 

 remains thin and simple for a long time. With the elongation of 

 the jaws the tip of the tongue grows forward above the frenulum 

 (Fig. 148) and the shape of the entire organ conforms itself to 

 the shape of the mouth cavity. 



Figure 175 shows the tongue of the adult fowl. The anterior 

 half is pointed and horny and is bounded from the posterior half 

 by a double crescent whose posterior convexity is beset with horny 

 spines. It seems probable that the anterior portion is derived 

 from the precopular part, though this has not been demonstrated 

 by continuous observation. Cornification of the precopular part 



