DENTAL SYSTEM 17 
the second set are developed in a precisely similar manner to the: 
first, but the enamel-germ, instead of being derived directly from an 
independent part of the oral epithelium, is formed from a budding 
out of the neck of the germ of the tooth succeeded. In the case of 
the true molars, which have no predecessors, the germ of the first 
has an independent origin, but that of the others is derived from the 
neck of the germ of the tooth preceding it in the series. The 
foundations of the permanent teeth are thus laid as it were almost 
simultaneously with those of their predecessors, although they 
remain in many cases for years before they are developed into 
functional activity. 
Although the commencement of their formation takes place 
at an early period of embryonic life, teeth are in nearly all mam- 
mals still concealed beneath the gum at the time of birth. The 
period of eruption, or “cutting” of the teeth as it is called, that is, 
their piercing through and rising above the surface of the mucous 
membrane, varies much in different species. In some, as Seals, the 
whole series of teeth appears almost simultaneously; but more often 
there are considerable intervals between the appearance of the 
individual teeth, the front ones usually coming into place first, and 
those at the back of the mouth at a later period. 
Forms of Teeth.—The simplest form of tooth may be exemplified 
on a large scale by the tusk of the Elephant (Fig. 1,1) Itisa 
hard mass almost entirely composed of dentine, of a conical shape 
at first, but during growth becoming more and more cylindrical or 
uniform in width. The enamel-covering, present on the apex in 
its earliest condition, soon disappears, but a thin layer of cement 
covers the circumference of the tooth throughout life. In section 
it will be seen that the basal portion is hollow, and contains a large 
conical pulp, as broad at the base as the tooth itself, and deeply 
imbedded in the bottom of a recess, or socket, in the maxillary 
bone. This pulp continues to grow during the lifetime of the 
animal, and at the same time is converted at its surface into dentine. 
The tooth therefore continually elongates, but the use to which the 
animal subjects it in its natural state causes the apex to wear away, 
at a rate generally proportionate to the growth at the base, other- 
wise it would become of inconvenient length and weight. Such 
teeth of indefinite growth are said to be “rootless,” or to have 
“persistent pulps.” 
One of the corresponding front teeth of man (Fig. 2, II. and III.) 
may be taken as an example of a very different condition. After its 
crown is fully formed by calcification of the germ, the pulp, though 
continuing to elongate, begins to contract in diameter; a neck or 
slight constriction is formed ; and the remainder of the pulp is con- 
verted into the root (often, but incorrectly, called “fang”), a taper- 
ing conical process imbedded in the alveolar cavity of the bone, and 
2 
