ON THK DEVELOPMENT OF THE TEETH 2^^ 



the " organon adamantince " adheres to it. In any case the adhesion 

 is loose, and these cells may be readily detached. Under a higher 

 power the surface of the upper part of the ossified cap appears 

 reticulated, the meshes being about i-5oooth of an inch in diameter. 

 At the lower part, where only a thin layer of dentine is formed, this 

 appearance is less distinct, but the surface is somewhat wrinkled the 

 wrinkles sometimes forming large and pretty regular meshes. Viewed 

 in profile, these wrinkles are seen to be produced by the folding of a 

 delicate structureless membrane, which is continuous below with the 

 membrana preforviativa. Towards the apex the tooth substance is 

 almost too opaque to make much out of it ; the yellowish enamel, 

 however, can generally be distinguished from the dentine. 



Now, while the object is under a low power of the microscope, add 

 some strong acetic acid ; a voluminous transparent membrane will 

 immediately be raised up in large folds from the whole surface of the 

 tooth. If the acetic acid be pretty strong, it soon softens the substance 

 of the tooth a little, and then a slight pressure exhibits very distinctly 

 the ends of the enamel fibres under this vievibrane. There can be no 

 question about this fact, as I have been able to demonstrate it to the 

 satisfaction of my friends, Mr. Busk and Professor Quekett. The 

 membrane is about 1-2 500th to i-i6ooth of an inch thick, perfectly 

 clear and transparent, and under a high power exhibits innumerable 

 little ridges upon its outer surface, which bound spaces sometimes 

 oval and sometimes quadrangular, and about i -5000th of an inch in 

 diameter. Furthermore, at its lower edge this membrane gradually 

 loses all structure, and passes into the membrana preforviativa} In 

 fact, it is the altered membrana preforviativa itself no trace of \\'hich 

 has ever yet been found in the locality in which, according to the 

 prevalent h5'potheses upon the development of the teeth, it should 

 exist — viz., between the enamel and the dentine. 



In the Calf ^ a similar membrane may be demonstrated, but it is 

 much more delicate, and I have not seen the peculiar areolae upon its 

 surface. 



In the Frog, in which the layer of enamel is very thin and struc- 

 tureless, the membrane {fig. 8) may be very readily demonstrated by 

 the action of dilute hydrochloric acid, which in this animal, as in the 

 Mackerel and Skate, dissolves out the enamel layer at once, while it 

 only acts gradually upon the dentine. 



1 It is stated, by all the writers on the subject whom I have consulted, that the membrana 

 pretormativa is the first portion of the tooth which ossifies. This statement, however, is 

 never supported by evidence ; and my own observations lead to precisely the reverse 

 -conclusions. ^ ^^^ Hassall, Micr. Anatomy, p. 31S. 



