INTRODUCTION. lix 



delicate nucleoli into compartments ; or that the remains of such 

 multiplied nucleoli may cause a modification of the walls of the 

 cyhnder, and so produce the characteristic transverse striae of the 

 enamel-fibre. This appearance is not present in the enamel of 

 the Frog's tooth, nor in that of the teeth of the Hog, or Calf, in 

 which animals my observations of the development of this tissue 

 have been chiefly made. As the development proceeds, the cells in 

 immediate contiguity with the calcified prisms undergo the same 

 changes as their predecessors, and become united to them by their 

 peripheral pointed extremities, whilst the fluid plasmatic contents 

 of the cells are exchanged for the dense salts of which the enamel 

 is chiefly composed. The selective surface formed by the organic 

 membrane of the cell would seem to be destroyed by the very 

 pressure resulting from its own action, and exerted by the contents 

 of the closely-packed contiguous prisms, when the cavities of the 

 cells are completely filled. The membrane ceases at least to be 

 distinguishable under the microscope, from the solid contents of 

 the cell, except at that surface of the enamel next the capsule, 

 and which is still in progress of growth, (as in PI. 123, fig. 2.) 



What is remarkable, here, is that not the whole of the actinen- 

 chymatous part of the enamel-pulp is converted into the long and 

 slender prismatic cellular basis of the enamel; at least, in the 

 valleys of the complex crown of the molar of the Ruminant and 

 Pachyderm, (Calf and Colt), this part of the enamel-pulp originally 

 occupies more space than the subsequent layer of enamel does: 

 and the superfluous peripheral part seems to be absorbed, and its 

 place to be occupied by a growth or thickening of the vascular 

 capsule. No capillaries pass from the capsule into the actinen- 

 chymatous pulp of the enamel : nor have I been able to trace a 

 blood vessel into that part of the capsule which was actually the 



e 2 



