402 
ME. S. J. A. SALTEE ON THE STEHCTUEE AND 
Enamel Bods . — The most difficult point of investigation in the structm’e of the 
Echinus-tooth is the method of development of the enamel, and its relation to the other 
elementary parts of the organ. This arises from two causes — the smallness of the 
amount of tlie tissue itself, and the lateness of its appearance on the tooth, after the 
organ has become very dense and opake. 
Upon viewing the dorsal aspect of a set of primary plates, it will be seen that the 
face of each plate is covered by its predecessor, excepting for a small extent of its basal 
margin (Plate VII. fig. I), and it is from this small extent of exposed plate that the 
rods originate which constitute the enamel. When, by a fortunate fracture, some of the 
plates on which this growth has commenced are isolated and present their dorsal aspect 
to the observer, they are seen studded, for just that amount of their margin which is 
naturally exposed, with minute projections of carbonate of lime closely resembling the 
early formation of the soldering particles, but, unlike them, they are limited to a par- 
ticular and defined boundary. They difier, moreover, as will be seen by examining the 
more advanced of them, in becoming elongated, whereas the soldering particles as they 
grow increase in size mainly by circumferential enlargement. 
In figure 4, Plate VII. are represented fifteen primary plates, exhibiting their dorsal 
aspect with the enamel rods just commencing to be forme'd ; and as some of them are 
partially dislocated and pushed aside, it will be seen how circumscribed is the develop- 
ment of these enamel-rod growths, and how it is limited to just so much of the back 
surface of the plates as would be uncovered when in undisturbed position — those 
portions which, by the imbrication of the previous plate, or by the overlapping at the 
central line of junction of the plates of the opposite side, would be covered up, being 
free from these developments. The fibres, which constitute the enamel, grow back- 
wards nearly horizontally, and by their lateral union among themselves form a compact 
structure, leaUng simple and narrow tubular intervals. 
Soldering Particles . — Up to a certain limit in the progress of development, the primary 
plates and the supplemental elements that enter into the structure of the tooth are free, 
excepting that the latter are attached to the former by their points of origin — the keel 
fibres, the flabelliform processes, and the enamel rods adhering to the primary plates 
by their proximal extremities. All the elementary parts themselves, excepting these 
attachments, though packed in close contiguity and entangled together, are free and 
fioating in the circumambient fiuid of the developmental sac. 
A fresh formation now occurs of a very remarkable nature. 
Scattered over the surface of all the elementary parts of the tooth there appear count- 
less multitudes of minute points of carbonate of lime ; at first these are the smallest 
microscopic specks, such as are depicted on the keel rod at Plate VI. fig. 7, h. These 
particles adhere firmly to the surface upon which they grow, and increase both in circum- 
ference and thiclaiess, the former, however, more than the latter. The increase of 
growth of these calcareous points' continues till the contiguous elements of the tooth 
are by them soldered together. It is from this circumstance, in accordance with the 
