PROGRESSIVE DEVELOPMENT. 39 



the papillae grew, they showed peculiarities of form 

 corresponding with that of the crowns of the future 

 teeth. After a time the lips of the dental groove and 

 small lamina? or opercula of membrane developed from 

 the sides of each follicle begin to cohere from behind 

 forwards, obliterating the groove and converting the 

 follicles into closed sacs, and then commences the 

 saccular stage of the milk-teeth. 



As these stages proceeded, Goodsir observed seve- 

 ral changes in the growth of the human tooth reseni- 

 bling what is met with in the lower animals, and felt 

 disposed to view the opercula of the human tooth fol- 

 licle as rudimentary organs, which attain their utmost 

 development in the sacs of the elephantoid, ruminant, 

 and other compound teeth, under the form of depend- 

 ing folds for the secretion of the intersecting enamel and 

 cement-plates. As illustrative of the law of progressive 

 development, he cited the human molar tooth-pulp — 

 first rounded as in certain fishes ; then conical as in 

 other fishes and reptiles ; then conical and flattened 

 transversely as in the carnivora; and finally, by the 

 equalisation of the primary and secondary tubercles 

 assuming the shape of the molars in the quadrumanous 

 animals and man. 



Wlii'ii the primitive dental groove contained ten 

 appillse in as many follicles, and became situated on a 

 higher level than at first, he denominated it the s< cond- 

 ary denUd yroorc. This groove affords a provision 



for the production of all the permanenl teeth, with the 

 exception of the first anterior molars : the preparation 



