42 ON THE ORIGIN AND DEVELOPMENT OF THE 



development of the human body. During the eighth week 

 the elevation already mentioned becomes a papilla, length- 

 ened from behind forwards, and flattened transversely. About 

 the same time another papilla, bounded by a notched lamina, 

 similar to those on the upper jaw, makes its appearance 

 further forward in the groove. This papilla is the germ of 

 the inferior milk-canine. The dental groove is about the 

 same time continued forward to the median line, not by the 

 advancement of its outer ridge, but by the elevation of its 

 floor. Its posterior portion also has become wider and not so 

 curved. 



During the succeeding week the incisives make their 

 appearance, the centrals first. 



From this time all the eight papillae continue to increase. 

 The notched laminae shoot inwards to the inner lip of the 

 groove, near which they meet and join slight projections from 

 it. About the eleventh or twelfth week the germ of the 

 posterior milk-molar appears in the curved portion of the 

 groove, and is developed in the usual manner. 



Crescent-like depressions appear in the secondary groove, 

 on the inner side of the mouths of the milk-follicles, as in the 

 upper jaw. 



The secondary groove adheres, leaving a posterior open 

 portion, in which are developed the papilla and follicle of the 

 first permanent molar. This follicle closes, as well as the lips 

 of the portion of groove above it. There are now in the jaw 

 ten milk-tooth sacs, two permanent-tooth sacs, ten anterior 

 cavities of reserve, and two great or posterior cavities of 

 reserve ;* the ten anterior for the development of the incisives, 



* The mucous membrane constituting the cavities of reserve exists in a 

 condition which has hitherto been considered by anatomists as peculiar to the 

 serous membranes. A dental cavity of reserve is a shut sac, lined by a true 

 mucous membrane, which is isolated from the general mucous system, and per- 

 forms no special function, till it is called upon to supply what it alone can 

 afford, materials for the development of a tooth. 



