38 The Microscope. 



elsewhere among mammals. We will notice briefly the organ as 

 a whole, and separately the outcome of each class of cells. 



As a combination the enamel organ sinks deeper into the tis- 

 sues, and the neck of the cord grows smaller and narrower until 

 finally it separates from the mother epiblastic layer entirely. 

 The organ is now wholly beneath the surface where it is entirely 

 surrounded by mesoblastic cells, as illustrated in Fig. 6. The 

 dental papilla pushes its way onward and upward carrying the 

 two epiblastic layers, now known as the outer and the inner 

 tunics, nearer together at the apex of the tooth. 



A moment's reflection will substantiate the fact that the cells 

 which are enclosed by the tunics are simply epithelial cells de- 

 rived orginally from the mucous membrane of the mouth, lying 

 as they did originally upon the surface and being a part of the 

 epiblast. In the original from which Fig. 6 was taken, the cells 

 around the inner edge next to the tunic, as at k, Fig. 6, are dis- 

 tinctly epithelial in appearance, while those in the interior, as 

 at j, are changed to a stellate form ; this is now known as the 

 stellate reticulum. But it is at the point where the papilla and 

 the inner tunic of the epiblast are striving for the mastery of 

 position, so to speak, that the most wonderful changes occur. 



Between the surface of the enamel organ and the dental pa^ 

 pilla, no union occurs ; simply one surface is perfectly adapted 

 to the other. Authority states, that " Vessels or nerves nave 

 never been demonstrated to pass from one to the other," and 

 from the fact that the enamel and dentine, both of which are pro- 

 ducts of these organs, can be very easily separated from each 

 other, it is proved that no union was ever established. 



At this point the shape of the cells which lie in opposition, as 

 at g, h. Fig. 6, or o, s. Fig. 7, are changed very materiall3\ They 

 become crowded together into line, losing their oval form and 

 becoming columnar or prismatic. 



The cells of the inner tunic are now known as the ameloblasts, 

 while the terminal ones of the dentinal papilla are known as 

 odontoblasts. Between these layers of cells are formed the den- 

 tine belonging to the odontoblasts while the enamel is formed 

 under the superintendence of the ameloblasts. (See Fig. 8 a, 

 and o). 



Up to this time we have only considered the formation and 

 development of the temporary tooth. The permanent tooth is 



