MUSEUM OF COMPAKATIVE ZOOLOGY. 249 



disregards the existence of the dental lamina and calls it the enamel 

 germ, of which at this stage I find no trace. He also figures (Taf. III. 

 Fig. 1-9), as existing throughout the incisor and canine regions, an 

 area — or, as Tomes calls it, a "halo" — in the mesoderm which iu 

 his opinion always precedes the development of the dentine germ, and 

 is probably caused by a concentration of mesodermic nuclei. I have 

 been unable to find, in. any of the stages of development, this halo 

 existing in any region of the upper jaw farther forward than that of the 

 first premolar. The only indication of sxich a condition has been a 

 slightly increased concentration in the nuclei immediately surrounding 

 the epithelial germ, which, however, is hardly more pronounced than in 

 the mesoderm which abuts upon neighboring portions of the buccal 

 epithelium, and which is never sharply marked ofl;' on the deep side 

 from other concentrations of nuclei which presage the formation of 

 cartilage in the deeper mesoderm. 



In an embryo 56 mm. long, cut in the same manner as the one just 

 mentioned, the dental lamina is present throughout the incisor region. 

 When studied with a low magnifying power (x 33), the sections through 

 a part of the region of the first incisor, as in the younger stage, show 

 no differentiation of a lamina from the plunging wall (figs. ll-H) ; but 

 when a higher power is used (x 175), the malpighian layer of that por- 

 tion of the plunging wall from which the lamina ought to proceed, is 

 found to present an irregular outline on the side toward the mesoderm 

 (fig. 30, I. big.). It is no longer a layer one cell deep, but is composed 

 of irregularly arranged cells, several deep. In the sections further 

 back, behind the middle of the first incisor, and throughout the remain- 

 ing incisor region (figs. 15-19), the dental lamina can be distinguished 

 from the plunging wall by its size and direction. Its malpighian layer 

 on the lingual side has undergone the same change as that just de- 

 scribed in the region of the first incisor. The histological condition of 

 the lamina in this region is identical with that which is presented at an 

 early stage of development by the lamina, in regions where teeth are 

 normally produced.* 



* I find that the two walls of the dental lamina in the premolar region of tlie 

 upper jaw, and in the incisor, canine, and premolar regions of the lower jaw, are 

 histologically different. Pouchet and Chahry have mentioned this fact (pp. 154- 

 155), and have given the names "adamantine" and " abadamantine " to tiie two 

 walls of the lamina; but they do not state what the structural difference is. I 

 have found that the malpighian layer of the lingual wall of the lamina is several 

 cells deep. These cells are prismatic in form, and are compactly though irregularly 



