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 papillae 

 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- 



