294 
ME. C. S. TOMES ON THE DEVELOPMENT OE THE TEETH 
loped in the midst of mature structures, whereas the tooth-sac of Mammalia arises in 
the midst of embryonic tissue. 
The substantial identity of the dentine-papilla and of such capsule as exists is well 
shown in fig. 20, where the structures going to form the capsule are continuous and 
blended with the forming dentine-papilla ; while above this the perfectly distinct origin 
of the enamel-organ from an exceedingly elongated process of the oral epithelium is 
clearly seen. 
A comparison of the tooth-sacs of the newt, frog, and lizard shows many points of 
close resemblance, the most noteworthy difference being in the extent to which a 
capsule is derived from the base of the dentine-papilla. In the newt the dentine-papilla 
ends abruptly, contributing absolutely nothing to the formation of a capsule external to 
the enamel-organ, so that the tooth-“ sac” is devoid of a capsule (Plate 46. fig. 6) ; in 
the frog it does appear to take some very slight share in the formation of an imperfectly 
defined capsule (Plate 47. fig. 15) ; while in the lizard it is distinctly continuous with a 
sort of capsule (Plate 47. fig. 19), which is recognizable at all stages of the development 
of the tooth. 
In this respect, - therefore, as also in the structure of the tooth pulp, the lizard 
approximates more closely to the structure of the mammalian tooth-sac than do the 
others. 
The much vexed general question as to the existence of a “membrana prseformativa ” 
can be more profitably discussed when our knowledge of the tooth-sacs of fishes is more 
definite ; but nevertheless a few words about it may not be out of place here. 
A “ membrana praeformativa,” in the sense in which the older writers used the term, 
viz. as a membrane covering the “ dental papilla” in common with the rest of the 
surface of the mucous membrane, clearly cannot be said to exist, seeing that the changes 
resulting in the formation of a dentine-papilla take place far below the surface, in the 
solid substance, so to speak, of the connective tissue. If there is at any time a membrane 
proper to the dental papilla, it is a special subsequent formation, having nothing to do 
with the basement membrane, and is in its origin quite a different thing from the mem- 
brana praeformativa as originally conceived. 
It is quite possible, however, that the offshoot from the oral epithelium may carry 
down in front of it during its descent into the submucous tissue a pouch of basement 
membrane, which would in this case intervene between the enamel-cells and the dentine- 
papilla, though it would belong to the former rather than to the latter. Although 
there would seem to be an a priori probability in this supposition, the appearances pre- 
sented by the epithelial processes in the frog (Plate 47. fig. 14) do not favour the suppo- 
sition that they are bounded by a membrane ; they are distorted and destroyed by very 
slight pressure or very slight pencilling ; and in the case of the newt, after a tooth-sac 
and its surroundings are broken up by pressure, I can discover nothing whatever but 
cells. 
And, again, the manner in which the connective tissue outside the area of tooth- 
