TOMES, ON THE DEVELOPMENT OF THE ENAMEL. 217 



tremely delicate sheath of the enamel fibre would under such 

 treatment soon disappear, and he might have got rid of the so- 

 called membrane by a continued application of the same agent, 

 in which case he might as fairly have argued that no soft 

 tissue existed, as he has done in assuming that a distinct 

 membrane bounds the enamel fibres because the sheaths liave 

 been dissolved by an alkali, before the partly-ossified distal 

 extremities disappeared.* 



The appearances which I have described, as existing in one 

 specimen, may be found in the teeth of similar age in any 

 fcetus, which has not been too long kept. Immersion in spirits 

 of wine for a short time, I think, favours the demonstration, 

 as the extremely-delicate columns of the enamel organ become 

 hardened, and hence keep their normal position more fre- 

 quently, than in perfectly-fresh subjects. Still in tUe latter 

 similar structural conditions to those 1 have described may 

 be observed. 



If, instead of taking an incisor, the first molar of a nine- 

 months' foetus be selected, the tooth-sac will be found dis- 

 tended with a fluid, in which numerous nucleated cells float. 

 Generally the cusps of the pulp are covered by taps of den- 

 tine, though this is not uniformly the case at this age. In 

 several instances I have preserved specimens, in which one 

 cusp only was invested with dentine, while the others were 

 quite free from calcification. In the latter case the membrana 

 preformativa should be distinctly visible. I have not been 

 able, however, to see anything that conveys to my mind the 

 idea of a distinct and separable membrane. A slight amount 

 of transparent tissue may be seen extending beyond the peri- 

 pheral dentinal cells, but it also dips in between them, and 

 has all the appearance of being nothing more than the blas- 



* The results of the following experhiient illustrate the amount of 

 dependence which can be placed upon membranes, the existence of which 

 cannot be deraoustrated otherwise than by the use of reagents. A thin 

 longitudinal section was prepared from the upper incisor of a rat. 1'his 

 was placed for a short time in hydrochloric acid and water (one part of 

 acid to eleven parts of water) ; on removal, the acid was neutralized by a 

 solution of potash. When placed in the field of the microscope, it was 

 seen that membranes had started up from the whole surface of the prepa- 

 ration. Not onlj' did a membrane part from the surface of the enarnel, 

 but one equally distinct peeled up from the worn, masticating surface of 

 the tooih, while others appeared upon the surfaces which were jiroduced in 

 grinding the section. The membranes thus demonstrated were distinct, 

 clear, and trausiiarent, but exhibited no trace of the structural characters 

 of the tissues from which they were derived, and of which they liad 

 formed a part prior to the application of the reagents. In this cxiieriment, 

 the action of the acid was arrested by the potash before the whole of tlie 

 section had been decalcified. The edges and surfaces were softened, but 

 the interior remained firm and retained its structural characters. 



