42 CHARLKS S. TOMES. 



out how Goodsir may be supposed to have fallen into the 

 error involved in his description. 



If a fcetus be kept in spirit the epithelium is very apt to 

 peel off, or to become but loosely adherent. A very slight 

 amount of manipulation would tear open the line of inflected 

 epithelium, thus exposing at its bottom the eminences form- 

 ing the dentine pulps. These would thus be described as 

 free, uncovered papillae at the bottom of a deep groove ; the 

 error of description is thus very intelligible, and was hardly 

 to be avoided at that date, when the methods of microscopic 

 research were in their infancy. 



At this stage we have distinguishable the enamel organ and 

 the dentine pulp, an indication of the future dental sac being 

 visible in the form of prolongations from the base of the 

 dentine pulp, which pass up outside the enamel organ 

 (see h in fig. 1). The latter, formed as it was from an 

 ingrowth of the cells of the rete Malpighi, retains for some 

 time its connection with the oral epithelium through a narrow 

 band of cells called the " neck of the enamel organ " (c in 

 figs. 1, 2, &c.). 



After awhile the neck of the enamel organ becomes 

 broken, and its connection with the oral epithelium lost, 

 while the tooth-sac is completed by the upgrowths from the 

 base of the dentine papilla before alluded to (A in fig. 1), arch- 

 ing over the top of the enamel organ and meeting above it. 



In the meantime the enamel organ itself has become modi- 

 fied, so that it now consists of a continuous sheet of large 

 cells, extending round its periphery, while its interior has 

 become transformed into a "reticulum^' of stellate cells (/in 

 fig. 1). The peripheral epithelial cells, where in contact 

 with the dentine papilla, form a regular pavement of columnar 

 or prismatic cells (enamel cells, e in the figures), and take the 

 name of " internal epithelium of the enamel organ;" while 

 those forming the outer wall of the enamel organ are more 

 rounded, and go by the name of the " external epithelium of 

 the enamel organJ'^ 



The enamel is formed by the direct calcification of the 

 cells of the internal epithelium of the enamel organ, the share 

 taken by the other portions of the enamel organ being com- 

 paratively insignificant. 



Thus far the origin of the tooth-sacs of the temporary 

 teeth only has been alluded to ; the permanent tooth-sacs, 

 originating at a much earlier period than has generally been 

 supposed, are developed in a precisely analogous manner, 

 save that the epithelial processes or enamel-germs whence 

 are formed their enamel organs do not spring directly from 



