ALIMENTARY TRACT AND ITS APPENDAGES 



303 



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the mouth: the egg-tooth is a mammiform hard structure with 

 pointed nipple (Figs. 176 and 177) situated on the dorsum of the 

 upper jaw near its tip (cf. Fig. 150). 

 Its function is to aid in breaking 

 the shell-membrane and the shell it- 

 self at the time of hatching; shortly 

 afterwards it is lost. It is, there- 

 fore, an organ concerned with a sin- 

 gle critical event in the life of the 

 individual; nevertheless fully de- 

 veloped like the instinct of its use, 

 needed only for the same critical 

 event. Though its structure is dif- 

 ferent from that of the beak, it de- 

 velops in connection with the latter, 

 and the two will, therefore, be con- 



■s^- 



Fig. 176. — Outline of the up- 

 per jaw of a chick embryo 

 of 18 days' incubation. (After 

 Gardiner.) 



E. T., Egg tooth. L. gr., Lip 

 groove. 



sidered together. 



The formation of the egg-tooth begins on the sixth day from 

 an area situated in the middle line near the tip of the upper jaw, 

 distinguishable in the living embryo by its opacity, which con- 

 trasts with the translucency of 

 the surrounding parts; in pro- 

 file view, the area is seen to be 

 slightly elevated. In sections 

 the appearance is found to be 

 due to an accumulation of 

 rounded ectodermal cells lying 

 between a superficial layer of 

 periderm of several layers of 

 cells, and the subjacent mucous 

 layer of the epidermis (Fig. 

 177). Without losing their 

 rounded shapes this mass of 

 cells gradually assumes the 

 form of the egg-tooth by the 

 fourteenth day. The overlying 

 layer of periderm is lost during 

 the act of hatching. During their differentiation the cells of the 

 egg-tooth secrete an intercellular substance of horny consistency 

 in which intercellular protoplasmic connections are found. The 



Fig. 177. — Transverse section through 



the upper jaw of a chick embryo of 



11 days. (After Gardiner.) 



E. T., Egg tooth. H. Horn. L. gr., 

 Lip groove. Pd., Periderm. T. R., 

 Tooth ridge. 



