A. M. Spurgin 79 



the jaw at this point being 1.5 mm. The incisors were separated from 

 each other by about .5 mm. These rudimentary incisors diminished in 

 size and degree of development from behind forward as shown in Plate 

 II, Fig. 1. They were separated by connective tissue with the exception 

 of the last two, which were separated by a rather large piece of cartilage. 

 A similar piece of cartilage behind the last tooth and a somewhat smaller 

 piece growing up between the third and fourth teeth (Plate II, Pig. I), 

 would seem to indicate that sockets were to be formed for at least the 

 last two. The shape of the first three teeth is that of a true incisor 

 with a single cutting edge, while the shape of the last two is nearly that 

 of a typical cuspid with a single somewhat prominent cusp. 



On each of the rudimentary incisors a layer of enamel has been depos- 

 ited. The relative thickness, which diminishes from the back tooth for- 

 ward, is represented in Plate II, Fig. 1, by the black line. Under the low 

 power, the enamel appears as a dark band which, in many sections, has 

 been pulled away from the dentine and fractured in the direction of 

 the enamel rods. This was due to the sectioning, since the tissue had 

 not been completely decalcified by the Miiller's fiuid. Plate II, Fig, 2, 

 shows this condition in a high power drawing of one of the incisors. 

 With they'g-inch oil immersion lens the direction and structure of the 

 enamel rods could be made out. In the fourth and fifth incisors the 

 inner layer of the enamel organ had lost its columnar character; the 

 stellate reticulum had disappeared, and only a few layers of flattened 

 epithelial cells remained over the enamel layer. In the first three teeth, 

 in which the enamel was not so thick, more of the enamel organ re- 

 mained, and at places away from the central area of the cusp, the 

 columnar cells of the inner layer could be seen. The cells in the imme- 

 diate area of the cusps were flattened as in the case of the last two teeth. 

 This clearly indicates that the enamel in the last two teeth has been 

 completely laid down, while more may yet be deposited from the col- 

 umnar cells in the three anterior teeth. No Nasmyth's membrane could 

 be found, and no secondary buds were observed in any of the rudi- 

 mentary incisors. These buds were not to be expected, since the develop- 

 ment had advanced considerably further than in the 6 cm. embryo of 

 Dasypus hyhridus, in which Eose demonstrated their occurrence. Al- 

 though I carefully examined the sections from the upper jaws of both 

 embryos, I failed to find any trace of Imds for rudimentary incisors. 



From a study of the longitudinal and transverse sections from the 

 lower jaw of the 8.5 cm. embryo, it could be seen that the tooth-buds 

 of the eight back teeth were almost completely surrounded by cartilage. 

 The two plates of cartilage forming the groove in which the teeth were 



