Parts of the Lantern of Aristotle in Echinus miliaris. 473 



Four stages may be recognised in its construction : — 



(1) The formation of a pair of primordial lamellae ; 



(2) The deposition of secondary pairs of lamellae ; 



(3) The consolidation of all these ; and 



(4) The attainment of a relatively stable position by the permanently 



active tooth-germs which will henceforth constitute the root-end.* 



The earliest traces of a tooth, so far as I have made out, are an incipient 

 cone, incipient in the sense that, not being completely closed on the outer 

 side, it is a cone in making. It will be seen presently that each cone which 

 comes into being subsequently arises from a pair of lamellae. One may, 

 therefore, reasonably infer that each of these five cones also arises in the same 

 way from a pair of lamellae. We shall accordingly call these five cones, the 

 " first cones," and the pair of lamellae of which each first cone is believed to 

 be composed, the " pair of primordial lamellae." Each first cone is situated in 

 an inter-radius (Plate 11, fig. 1, F.G.), with its apex turned towards the future 

 oral aspect and its base directed towards the future aboral aspect. This cone 

 serves as a starting-point for the upbuilding of the entire tooth ; for within 

 the cavity of this cone in the metamorphosing pluteus are found deposited 

 about six pairs of secondary lamellae. The first cone remains as a distinct 

 structure, even after a good bit of the embryonic tooth has come into being 

 (text-fig. 3, F.G.); it is highly probable that finally it enters into union with 

 its successor and forms the tip of the embryonic tooth. 



A study of the numerous pairs of lamellae deposited in succession after 

 the first cone has been formed reveals several facts of no small importance 

 and interest. Each lamella is laid down as a round calcareous particle, which, 

 as I have evidence to show, is probably the product of a single calciferous 

 cell. This particle grows by accretion of lime on one side only in such a way 

 that a small triangular plate is first formed (Plate 11, fig. 1, L, and text-fig. 1). 

 At the base of this plate a peripheral growth takes place in one plane, 

 resulting in an imperforate transparent lamella. When it has attained its 

 maximum vertical growth it has got the contour of an isosceles triangle, with 

 its apex directed towards the ventral aspect and its base towards the aboral 

 surface. Even before a particular lamella reaches its full size, fresh lamellae 

 are deposited at higher levels, one above the other, in quick and regular 

 succession, always on the inner side of the preceding lamella. One can, 

 therefore, read the several stages passed through by a single lamella from the 

 series of lamellae representing different stages in their growth (text-fig. 2). 



* This fact will be elucidated in a paper on the soft parts of the lantern, now in 

 preparation. 



