DEVELOPMENT OF THE TEETH IN MAMMALIA 31 



calcified skin scales the placoid scales of Elasmobranch 

 fish described by Hertwig (see p. 177). 



By the placoid stage of tooth development, Rose under- Placoid 

 stands the condition in which the earliest tooth germs in stage * 

 the region of the mucous membrane of the jaws develop and 

 proliferate over the deeper lying cells, as do the placoid 

 scales of the Sharks or the papillae of the skin in Mammalia. 



The epithelium of the jaw is somewhat thickened in 

 places, and beneath these thickenings the round cells of the 

 connective tissue have accumulated. 



In distinct circumscribed areas the growth of the epi- 

 thelium is more pronounced, and the papillae of the placoid 

 germs are seen as lenticular epithelial growths. As in 

 Mammalia, the epithelial tissue is the true form-determining 

 element of the tooth germ. 



He affirms that in Lepidosteus and the Salmonidae the 

 earliest placoid germs by further growth sink deeper and 

 deeper into the connective tissue of the jaw, and exhibit 

 an intermediate stage between the placoid and the mam- 

 malian form of tooth development. 



In this, which he calls the conical stage, the whole mucous conical 

 membrane of the jaw is concerned, and its epithelial layer Sta 8 e - 

 contributes for each tooth germ a. definite epithelial process, 

 which in the later-formed teeth dips deeply into the mucous 

 membrane, as do the epithelial sheaths of the hair follicles. 

 Rose (19 c, d) would thus describe three stages in tooth de- 

 velopment : 



1. The placoid stage. In this, the germs are seen as 

 upstanding papillae of the mucous membrane beneath 

 definite thickenings of the epithelial cells which invest 

 them. 



2. A second or conical stage in which the germs sink 

 deeply into the mucous membrane enclosed by a distinct 

 epithelial sheath, as seen in the human hair follicle, and the 

 whole surface of the mucous membrane of the jaw is con- 

 cerned in tooth formation. 



3. The tooth-band stage, as seen in Mammalia, where the 

 epithelial germs are given off from a definite tooth-band, 

 and it is this differentiated portion of the epithelium only 

 which is concerned in tooth formation. 



