304 THE DEVELOPMENT OF THE CHICK 
protoplasm of the cell-bodies themselves becomes densely packed 
with granules, apparently also of a horny nature, and the boun- 
daries of the cells and outlines of the nuclei become indistinct. 
Reptiles with a horny egg-shell are provided with a true dentinal 
tooth on the premaxilla, which has the same function as the egg-tooth 
of birds and of those reptiles that have a calcareous shell (crocodiles, 
turtles, and Trachydosaurus). The latter is, however, as we have 
seen, a horny structure, and therefore not a tooth morphologically. 
Rose therefore proposes the term “ Eischwiele”’ for the horny tooth- 
like structure, to distinguish it sharply from the real egg-tooth. 
The formation of the upper beak begins in the neighborhood 
of the egg-tooth and spreads towards the tip and the angle of 
the mouth. Similarly, in the lower Jaw the differentiation begins 
near the tip. It is a true process of cornification, that takes 
place beneath the periderm and involves many layers of cells. 
It is therefore preceded by a rapid multiplication of cells of the 
mucous layer of the epidermis. Soon after the appearance of 
the horn a groove appears a little distance above and parallel to 
the margin of the upper beak, extending from the anterior end a 
short distance backwards (Fig. 176). In sections, this appears 
as an invagination of the epidermis; a similar but shallower 
invagination appears on the lower beak. In the upper beak the 
lips of the invagination fuse together and thus close the groove; 
in the lower beak the groove flattens out and disappears. These 
grooves correspond in many respects to the grooves that form 
the lips of other vertebrates, and they may be interpreted as a 
phylogenic reminiscence of lip-formation. 
Teeth. All existing species of birds are toothless, but some 
of the most ancient fossil birds possessed well-developed teeth; 
it is natural, therefore, to expect that rudiments of teeth might 
be found in the embryos of some existing birds. In the early 
part of the nineteenth century some observers interpreted papillie 
on the margin of the jaws of certain young birds as rudimen- 
tary teeth; these were, however, shown to be horny formations, 
and therefore not even remotely related to teeth. Gardiner was 
one of the first to call attention to a thickening of the ecto- 
derm forming a ridge projecting slightly into the mesenchyme, 
just inside the margin of the jaw of chick embryos about. six 
days old (Fig. 177). The ridge disappears shortly after cornifica- 
tion sets in. Gardiner discussed the possibility of this represent- 
