THE MICROSCOPE. 



Jan. 



in the Animal Kingdom. They sliow a gradual transi- 

 tion from the ordinary reticulated structure of the shell 

 to the peculiar substance of wliicli teeth in higher ani- 

 mals is made. The tooth is sometimes dissected out and 

 mounted in balsam but it needs to be ground down some- 



what to permit the light to pass through it. It is then 

 seen that the base of the tooth is made of material like 

 the shell and the upper or older part is made of bone, 

 the intermediary part showing a gradual transition from 

 shell to bone. 



