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 very 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 WKallius 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 (ef. 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 
