MUSEUM OF COMPARATIVE ZOOLOGY. 251 



surrounded by mesoderm. Throughout the regions of the third incisor 

 and the canine tooth, small portions of the unchanged dental lamina 

 are becoming separated from the epithelium. The corneous cells form- 

 ing the centre of these small epithelial projections become vacuolated, 

 and probably disappear, leaving only the cells of the malpighian layer, 

 which, upon separating from the buccal epithelium, and becoming em- 

 bedded in the mesoderm, form insulated masses or knots, which at this 

 stage are very numerous. I have found an occasional knot of this kind 

 in other regions of both jaws ; but nowhere else have I seen so many as 

 in the regions just mentioned. Since similar knots or islands appear 

 during the degeneration of the neck of the enamel sac, I judge that 

 they are all produced by the active ingrowth of mesoderm ic tissue, and 

 are to a certain extent indications of a regressive development on the 

 part of the dental lamina. 



Plana (p. 220) likewise believes that the disappearance of the dental 

 lamina in later stages is accomplished by its transformation into such 

 islands. Legros and Magitot (p. 469) also describe similar epithelial 

 knots, which are the remains of the dental lamina in the ordinary devel- 

 opment of teeth. Although the epithelial knots found in the anterior 

 portion of the upper jaw at this period are analogous to those found 

 .where the enamel organ is well developed, they differ from the latter, 

 since when first formed they consist of prismatic cells from the mal- 

 pighian layer. These cells gradually assume a more rounded shape, 

 and the knots finally become lost in the surrounding mesodermic tissue ; 

 whereas the knots described by Legros and Magitot, although buds from 

 the malpighian layer of the lamina, are claimed by them never to be 

 made up of prismatic cells, as is that layer, but of small polyhedral cells 

 which resemble those in the centre of the lamina at an early stage. 

 The difference in appearance of these two kinds of epithelial knots is 

 due, I believe, to the fact that those in the superior incisor and canine 

 regions are the result of the rapid breaking up of the dental lamina, 

 which quickly sets them free from the buccal epithelium ; whereas 

 those in the region of normally developing teeth are formed more 

 slowly, and probably as the result of a more active cell proliferation 

 on the part of the epithelium. 



In the canine region also (fig. 32) the enamel germ is losing connec- 

 tion with the plunging wall, owing to the ingrowth of mesoderm, and 

 is slightly larger than the germ in the embryo 56 mm. long. 



In an embryo 93 mm. long the dental lamina has increased in size 

 (figs. 34, 35), but, aside from a more marked difference between the 



