ALIMENTARY TRACT AND ITS APPENDAGES 
303 
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 hear its tip (ef. 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- 
m2 
 ° nae | 
co 
» 
\ 
Lgr 
Fic. 176. — Outline of the up- 
per jaw of a chick embryo 
of 18 days’ incubation. (After 
Gardiner. ) 
EK. T., Ege tooth. 
ne rs 
L. gr., Lip 
sidered together. 
groove. 
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- 
Fic. 177.— Transverse section through 
the upper jaw of a chick embryo of 
11 days. (After Gardiner.) 
EK. T., Egg tooth. H.Horn. L.gr., 
Lip groove. Pd., Periderm. T. R., 
Tooth ridge. 
the act of hatching. 
trasts with the translucency of 
the surrounding parts; in pro- 
file view, the area is seen to be 
shightly 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 (Tig. 
177)... Wathout their 
rounded shapes this mass of 
cells gradually the 
form of the egg-tooth by the 
fourteenth day. The overlying 
layer of periderm is lost during 
losing 
assumes 
During their differentiation the cells of the 
egg-tooth secrete an intercellular substance of horny consistency 
in which intercellular protoplasmic connections are found. 
The 
