GEOWTH OE THE TOOTH OF ECHINHS. 
397 
ai’e, in fact, incremental lines, and they are chiefly conspicuous at the outer angle, where 
the most rapid change has of late been going on (see Plate VIL fig. 4). As has been 
already observed, these lines are best seen by employing bright sunlight and directing 
the reflector of the microscope obliquely to one side of the field of vision. 
The primary plates require very careful observation ; they are, as I have before 
remarked, the phjsiological axis of the tooth, and dependent on them and attached to 
them are all the supplemental elements which enter into the construction of the organ. 
The two series of primary plates have a definite and constant arrangement. As they 
are free in the little aqueous sac that contains them, merely imbricated one over the 
other without any attachment, it is difficult so to place them under the microscope as 
to avoid derangement of their relations. Careful observation, however, shows that, as 
regards the mathematical axis of the tooth, they are doubly oblique. Vertically they 
are oblique downwards and outwards, i. e. from the superior enteric angle to the infe- 
rior dorsal angle ; and the second obliquity — the horizontal — arises from the internal 
inferior angle being further removed from the enteric region than the external. This 
will be best understood by referring to Plate VI. figs. 2 & 3, J, which represent the line 
of vertical obliquity, and the Woodcut I. a, which indicates the horizontal obliquity. 
Woodcut I.* 
In examining the soft growing end of the tooth, care should be taken to ascertain 
which face (enteric or dorsal) is opposed to the objective, and the thin glass which is 
imposed upon the specimen should be placed upon it lightly and without pressure. 
Much pressure may flatten and spread out the plates so as to be seen only face-wise, 
while traction downwards of the upper angle may present them to view edgewise, as in 
the case of Valentin’s figure. A long series of observations will enable the micro- 
* Diagram indicating, in a transverse section of a tooth, the position of a, Primary Plates ; b, Secondary 
Plates ; d, Flabelliform Processes ; cc', Fibres of the Keel ; eee, Enamel Eods. 
3 i2 
