36 TOOTH-GERMS. 



imbedded, be exaDiined, the nucleated cells are seen, 

 closely aggregated around the calcified part, in con- 

 centric rows, the cells of which are further apart as 

 the rows recede from the field of calcification. Those 

 next the cement rest in cup-shaped cavities in the 

 periphery of the calcified part, just as the first calcified 

 cells of the thick cement which covers the crown of a 

 complex molar are lodged in cavities on the exterior of 

 the enamel. These exterior cavities of the cement are 

 formed by centrifugal extension of the calcifying pro- 

 cess in the blastema m which the cells are imbedded. 

 The calcareous salts penetrate in a clearer and more 

 compact state the cavity of the cell, but their progress 

 is arrested apparently by the nucleus, which maintains 

 an irregular area, partly occupied by the salts in a sub- 

 granular, opake condition, but chiefly concerned in 

 the reception and transit of the plasmatic fluid, which 

 enters and escapes by the minute tubes that are sub- 

 sequently developed from the nucleolar cavity as calci- 

 fication proceeds. 



^' The radiated cells or cavities thus formed are the 

 most common characteristic of the cement, but not the 

 constant one. The layer of the capsule which sur- 

 rounds the crowm of the human teetli and of the 

 simple teeth of quadrumana and carnivora, consists 

 simply of the granular blastema, without nucleated 

 cell?, and the radiated corpuscles are, consequently, liot 

 developed m the cement which results from its calci- 

 fication. In the thicker part of the inflected folds of 

 the capsule of the complex teeth of the herbivora, 

 traces of the vascularity of that part of the matrix are 

 persistent, the blastema calcifying around certain of 

 the capillaries, and forming the medullary canals. 

 The varieties of these canals are traversed bv minute 



