976 
the description refers to the development of a lower tooth., The upper teeth are of 
course developed in a manner exactly similar. 3 
THE DIGESTIVE SYSTEM. 
1. The first distinct evidence of the development of the teeth is to be found ina 
thickening of the mouth epithelium, at the site of the future gum, and a resulting down- 
growth of its deeper portion into the substance of the primitive jaw (Fig. 659, I). This 
I. Shows the downgrowth of 
the dental lamina D.L from 
the surface epithelium E 
and the beginning of the 
enamel germ E.G. 
II. Shows the further growth 
of the enamel germ and its 
invagination. 
III. The enamel germ is more 
invaginated, and its inner 
layer of cells becomes 
columnar. <A, the dental 
lamina, grows thinner, but 
near its posterior or lingual 
edge there is an enlarge- 
ment R.G which is the re- 
serve germ fora permanent 
tooth. The superticial cells 
of the dentine papilla P are 
becoming columnar. 
IV. The inner columnar cells 
of the enamel gerin (called 
enamel cells) A have formed 
acap of enamel En, inside 
which the superficial cells 
of the papilla, the odonto- 
blasts O, have formed a 
layer of dentine D. 
V. Shows a more advanced 
stage still. The deposit of 
dentine is extending down- 
wards, and enclosing the 
papilla to form the future 
pulp, in which a vessel V 
is seen. 
epithelial downgrowth is continued along 
the whole length of the gum, and is 
known as the dental lamina or tooth- 
band (Zahuileiste). On the under surface 
of the dental lamina there soon appears 
aw series of knob-like projections—one 
for each of the milk teeth (Fig. 659, IT.) 
—which are known as enamel germs or 
enamel organs. ‘These enamel organs 
are connected with the epithelium of the 
dental lamina by a constricted part, and 
although at first knob-like, they soon 
become bell-shaped owing to the in- 
vagination of the lower surface of the 
knobs, so that each may now be com- 
pared to an inverted egg-cup. 
2. As soon as the enamel organs begin 
to assume a cup-like shape, the cellular 
connective tissue of the jaw beneath 
erows up and projects into the cavity of 
the cup (Fig. 659, III.) in the form of a 
papilla—the dentine papilla (papilla 
dentis). The arrangement, pursuing 
our simile, may now be compared to an 
egg fitting into its cup—the dentine 
papilla representing the egg, and the 
enamel organ the cup (Fig. 659, IIT.). 
3. The two layers of cells which are 
thus brought in contact, namely, the 
epithelial cells lining the concavity of 
the enamel organ, and the superficial 
cells of the dentine papilla, become 
elongated or columnar, and : undergo 
other changes, preliminary to the pro- 
duction of the enamel by the former— 
which are now called enamel cells or 
ameloblasts—and the dentine by the 
latter, which are known as odontoblasts. 
4. The odontoblasts, that is the layer 
of columnar-shaped connective tissue cells 
lying on the surface of the dentine papilla, 
begin to form at their outer ends a layer 
of dentine (Fig. 659, IV.). Similarly, 
Fic. 659.—DIAGRAM TO ILLUSTRATE DEVELOPMENT the enamel cells lining the cup begin to 
or A Tooru. form at their inner surface a layer of 
A, Inner layer of enamel germ; B, Outer layer ; C, Remains of enamel on the top of the layer of dentine 
intermediate cells; D, Dentine; D.L, Dental lamina; E, ae 5 TV thi a4 ve Sous 
Epithelium; E.G, Enamel germ; Ey, Enamel; F, Dental (Fig. 659, I\ -); to which It adheres : in 
furrow; L.D, Labio-dental furrow; M, Connective tissue each case the deposit taking place first 
cells; O, Odontoblasts; P, Dentine papilla; R.G, Reserve B nach 
Peni av load evescel. at the summit of the tooth. 
5. The formation of dentine and 
enamel proceeds apace, the dentine increasing at the expense of the papilla, the enamel 
similarly encroaching on the cup or enamel organ ; and in each case the two layers of 
cells—odontoblasts and enamel cells—which produced the deposits, retiring eradually 
from one another, as the space between them becomes occupied by the newly-formed 
dentine and enamel respectively (Fig. 659, V.). 
The remains of the dentine papilla persist as the pulp of the tooth, which is covered 
even in the adult by the odontoblasts, and occupies the pulp cavity, ¢.c. the central part 
of the tooth to which the dentine formation has not extended. 
