396 
ME. S. J. A. SALTEE ON THE STEUCTIJEE AND 
mthout any definite arrangement, like segments of the ambulacral discs, portions of the 
spines, alveoli, &c. 
The continuous development of the tooth, by the incessant reproduction at its growing 
end of the elementary parts which constitute it, afibrds the anatomist an opportunity of 
examining the nature and form of those elementary parts, and of observing the synthe- 
tical process by which they combine to build up this curious and complicated structm’e. 
I purpose now to describe these elements in detail — in the order of then’ appearance 
and their value in combination. 
Primary Plates . — The tooth originates at its soft growing extremity by the develop- 
ment of two sets of triangular plates (Plate VI. fig. 5). Valentin saw, correctly figured 
and described, the early growth of the first elementary parts of the Echinus-tooth, 
though he failed altogether to understand the relation of these parts to the entire organ. 
These plates commence as mere spots, so minute as to be of no definable form. They 
constitute two parallel series of equal numbers, at first quite distinct and separated by 
a clear mterval, but afterwards, by the general enlargement of their area by marginal 
growth, they approximate each other in the mesial line, and at about the 50th to the 
70th plate they begin to intersect in a perfectly regular alternation up the centre — the 
overlapping increasing more and more as the size of the plates increases up to a certam 
point, beyond which the further enlargement appears to take place almost wholly 
towards the outer margins. 
The form of the plates is a modified triangle, but the sides, instead of being straight, 
are more or less convex or waved. The superior internal (enteric) and the inferior 
internal angles are mucronated, the former more than the latter. The mucronation 
consists m a thickened point which projects beyond the plate, and passing back upon 
its surface is lost in a gradual thinning expansion. 
The form of the plates is constantly undergoing modification from their earliest 
appearance till the latest time at which they are capable of isolation and separate 
inspection. At first the aberration from a nearly-equilateral triangle consists in a pro- 
longation of the outer and inferior angle : as the plate enlarges this becomes less con- 
spicuous, and then the inner side of the triangle advances towards its fellow of the 
opposite series, in an irregular wavy line, at the same time the sharp mucronated angles 
become rounded and lost, and the thickened lines on the face of the plate also disappear. 
In the further development of the plates their outline is entirely altered, the triangle is 
lost, and, principally by prolongation of the outer inferior angle, the plate is converted 
into a broad wavy band. Compare fig. 5, Plate VI., figs. 1 & 4, Plate VII. It is very 
difficult to obtain the plates when they have assumed this latter form, as this change 
only occurs in a condition of very advanced development, when the plates have usually 
become more or less united with the contiguous elements of the tooth. 
’While the plates are increasing in area they become denser, thicker, and there now 
appear upon them, when viewed in a suitable light, certain lines having the same con- 
tour as the outlines of the plates themselves : these are indications of progressive growth, 
