252 BULLETIN OF THE 



labial and lingual walls, it is not histologically different from the condi- 

 tion in the embryo whose length was 87 mm. In the canine region 

 (fig. 33) the enamel germ has increased in size, and has sunk deeper 

 into the mesoderm. The epithelial islands now consist exclusively of 

 somewhat rounded cells from the malpighian layer, the corneous epithe- 

 lium having disappeared from them. These knots by their juxtaposi- 

 tion form an almost unbroken series between the enamel germ and the 

 epithelial plunging wall. 



When the embryo has become 112 mm. long, the dental lamina in 

 the region of the first incisor cannot be distinguished from the plunging 

 wall. The cells of both the corneous and malpighian layers, which in 

 younger stages had become differentiated, have again attained the con- 

 dition of those of the plunging wall. In the region of the lateral 

 incisors (fig. 36) the dental lamina, though yet distinguishable, has 

 become much smaller and sinks less deeply into the mesoderm. Its 

 differentiated cells have decreased in size, — cells from the malpighian 

 and corneous layers closely resembling each other, — and its boundaries 

 are less easily discernible on account of the greater irregularity in the 

 arrangement of the malpighian cells. The enamel germ of the canine 

 tooth (fig. 37) has become at this stage much diminished in size, and 

 its cells are less specialized than in the preceding stages. Several of 

 its central cells appear vacuolated, and this suggests an explanation of 

 the manner in which it has suffered a reduction in size. 



From the preceding account it is evident, (1) That in the embryo 

 sheep at a certain stage of development, the dental lamina exists 

 throughout the canine and incisor regions of the upper jaw. Its an- 

 terior portion, which is the last to develop and the first to abort, does 

 not attain so prominent a condition as its lateral portion. After ad- 

 vancing in development for a time, it retrogrades and finally disappears. 

 (2) That in the canine region the dental lamina gives rise to an enamel 

 germ which never reaches a stage of functional activity ; for neither are 

 its central cells transformed into a stellate reticulum, nor do those of 

 the malpighian layer ever produce enamel, and in later stages both 

 disappear. 



In this region there is no trace of a dentine germ. The fact of the 

 existence in sheep of rudiments of such organs as usually result in the 

 formation of teeth, is of interest because it is one of those peculiar 

 structures for which it is difficult to account without the aid of the 

 theory of natural selection. From the observations here recorded, one 



