ON THE DEVELOPMENT OF TEETH. 49 



from a groove lined by epithelium, yet any description which 

 spoke of the successional teeth of a snake, or yet more of its 

 poison fangs, as being developed from free papillae in a 

 groove, would be somewhat wide of the mark. 



]\Iy observations have led me towards certain other ques- 

 tions of interest. In the first place, are we still to regard 

 dentine as the most, cementum as the next, and enamel as the 

 least constant of dental tissues ? According to the older 

 accounts of development such must almost necessarily have 

 been the case ; but is it really so .'' The Ophidia, at all events, 

 amongst whom cemeutum has been supposed to be of uni- 

 versal occurrence, have none, but have enamel instead (see 

 ' Phil. Trans.,' 1875), and I am inclined to think enamel at 

 least as constant, if not more so, than cementum. 



The true homologies of the whole enamel organ were first 

 pointed out by Professor Huxley in the valuable paper re- 

 ferred to ; with regard to the several parts of the enamel 

 organ, the reticulum appears to be pecuHar to the mammalia ; 

 at all events it is certainly not an essential part, as enamel 

 is formed in countless teeth in which it was never present. 

 Indeed, some recent observations of my own upon the develop- 

 ment of the poison fangs of snakes would seem to point to the 

 inference that it is essentially a retrograde metamorphosis of 

 a part of the enamel organ. It has already been mentioned 

 that the concidence between the position of the large enamel 

 cells in an enamel organ such as that of the eel (see fig. 10) 

 with that of the enamel cap afterwards formed would, so far 

 as it goes, support the conversion theory of the development 

 of enamel; the more so as it is generally possible to judge 

 from the extent to which this internal epithelium of the 

 enamel organ is developed, whether enamel will or will not 

 be deposited upon any tooth ; Hertwig, however, believes that 

 enamel is not formed by direct calcification of the enamel 

 cells, but by a secretion from them ; he also reasserts the 

 existence of the membrana preform ativa,^ but not, in my 

 opinion, upon satisfactory grounds. 



The development of cementum is a subject by no means 

 satisfactorily worked out ; how far it has relation to the 

 tooth-sac capsule, and how far to the periosteum and connec- 

 tive tissue outside the sac, I hope to be able to determine ; 

 but it appears to me that its presence is almost always asso- 



' After a careful review of the literature of the subject, and not a few 

 attempts to demonstrate it, I cannot think that the presence of a basement 

 membrane {M. preformativa) has been establislied, and the appearances de- 

 scribed are, to my thinking, capable of other interpretations more in accord, 

 with what I have myself seen. Waldeyer also denies its existence. 



VOL. XVI. D 



