30 ON THE ORIGIN AND DEVELOPMENT OF THE 



The inner lip of the dental groove (or the outer edge o1 

 the palate), which has been increasing for some time past, is 

 now, at the fourteenth week, so large as to meet and to apply 

 itself in a valvular manner to the outer lip or ridge, which 

 has also been increasing. The follicles at this time grow 

 faster than the papilke, so that the latter recede into the 

 former. The molar papillaj gradually acquire two or three 

 additional small compressed tubercles on their sides, and their 

 apices become less conical, so that they still more resemble 

 the molar teeth of the carnivorous mammals * The opercula 

 of the follicles continue to increase, so as almost to hide their 

 contained papilla?. 



The lyrimitive, dental groove, which at this period contains 

 ten papillse in as many follicles, and is situated on a higher 

 level than at first, may be now more properly denominated the 

 secondary dental groove. It is when in its secondary condition 

 that the groove affords a provision for the production of all 

 the permanent teeth, with the exception of the first or anterior 



tootli-follicles may not be rudimentaiy organs, which are to attain their utmost 

 development in the sacs of the elephantoid, ruminant, and other compoimd 

 teeth, under the form of depending fohls for the secretion of tlie intersecting 

 enamel and cement plates. 



One may easily conceive the mode of formation of a composite tooth-sac, 

 by supposing the opercula, after their edges have met, to dip down back to 

 back between the divisions of the pulp, till they almost meet the common 

 body of the latter. 



* This is another instance of the law of progressive development, by virtue 

 of which an organ, in the course of its formation, passes through phases which 

 correspond to permanent conditions of the same organ in other animals. A 

 human molar tooth-pulp is at first rounded, as in certain fishes ; then conical, 

 as in other fishes and reptiles ; then conical, but flattened transversely, gi-adu- 

 ally acquiring two or more additional conical tubercles, as in the carnivorous 

 mammals ; and finally, by the equalisation of the primary and secondaiy 

 tubercles, assuming the shape of the molars in the quadrumanous animals and 

 man. In the elephantoid, ruminant, and rodent animals, it probably under- 

 goes a further and ultimate change in the deepening of the rudimentary grooves 

 on the gi'inding surface. 



