GEOWTH OF THE TOOTH OF ECHTNHS. 
391 
The examination of the soft end of the growing tooth requires considerable adroit- 
ness. After removing the whole of the dental apparatus from a very fresh animal, 
the tooth, where it emerges internally from the alveoli, may be detached with the point 
of a knife, and itself freed from surrounding soft parts by means of fine-pointed scissors. 
When the growing end of the tooth is thus detached, it should be carefully drawn along 
the sm-face of a slightly moistened glass slide, just sufficient to extend it, the observer 
noting from the first whether it is the dorsal or enteric surface of the tooth which is 
uppermost and exposed to the objective. The specimen should then be treated with 
Liquor Sodae, which has the effect of completely clarifying the soft tissues accidentally 
attached, so that nothing is observed but the plates, fibres, &c. — the different tooth- 
elements, sharply defined and 'prononce. At the same time it will be found that dissec- 
tion of the tooth, where its structure has become complicated, is very difficult ; and it 
is only (in such reffions of the organ) by many repeated examinations, and by the clo'se 
and constant scrutiny of every detached portion of the disintegrated teeth, that happy 
and illustrative specimens can be obtained. 
One important element of the tooth-structure, the “ Soldering Particles,” to be 
described hereafter, can be best studied in those portions of the shaft about midway 
between its two extremities ; at a point, that is, immediately before they unite the 
se^’eral elements into a coherent mass. Little portions may be detached with needle- 
points, which display these particles adherent to the plates and fibres which they unite 
together ; and often parts of several plates and fibres may be seen thus united. 
Sections of the mature tooth are best examined by transmitted light — this ultimate 
histology with high powers ; the general structural arrangement with low. The soft 
growing end of the tooth should also be examined by transmitted light. In those cen- 
tral portions of the tooth, where very instructive fractures may be produced by slipping 
apart the as-yet loosely attached imbricated elements, the broken ends should be viewed 
with reflected light and a very low power, as a hand-lens. 
I have found that the nuclei and concentric rings in the soldering particles, and the 
incremental lines in the more advanced plates, can be best seen by a modification of 
transmitted light — by employing bright sunlight, but so adjusting the reflector as to 
direct the light just out of the limit of the field : in this partial darkness the appearances 
in question are most conspicuous. 
Before entering on a minute description of the histology and growth of the Echinus- 
tooth, I would premise a few general remarks which will render my subsequent explana- 
tions more intelligible. 
O 
The Echinus-tooth consists of an aggregation oi plates fibres of carbonate of lime, 
arranged in most curious complexity, but on a constant and definite plan. 
The tooth commences at the internal soft growing end by the formation of two series 
of thin triangular imbricated plates. These are physiologically, though not mathema- 
tically, the axis of the tooth, extending from end to end of the organ, and ultimately 
forming an important part of the body of the matured structure ; upon these plates. 
