DEVELOPMENT OF THE TEETH. 



1245 



the dental lamina or tooth-band. The dental lamina divides into two plates, a lateral 

 vertical and a medial more horizontal in direction. The medial plate is the portion 

 from which the teeth are formed, and is termed the dental ledge. On the under surface 

 of the dental ledge there soon appears a series of knob-like projections one for 

 each of the milk teeth (Fig. 971, II.) which are known as adamant germs or adamant 

 organs. These organs are connected with the epithelium of the dental ledge by a con- 

 stricted part, and although at first knob-like, they soon become bell-shaped owing to 

 the invagination of the lower surface of the knobs, so that each may now be compared to 

 an inverted egg-cup. 



2. As soon as the adamant organs begin to assume a cup-like shape, the cellular 

 connective tissue of the jaw beneath grows up and A 



projects into the cavity of the cup (Fig. 971, III.) 

 in the form of a papilla the papilla dentis. The 

 arrangement, pursuing our simile, may now be com- 

 pared to an egg fitting into its cup the papilla 

 representing the egg, and the adamant organ the cup 

 (Fig. 971, III.). 



3. The two layers of cells which are thus brought 

 in contact, namely, the epithelial cells lining the con- 

 cavity of the adamant organ, and the superficial cells 

 of the tooth papilla, become elongated or columnar, 

 and undergo other changes, preliminary to the pro- 

 duction of the adamant by the former which are 

 now called adamant cells or ameloblasts and the 

 ivory 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 dental papilla, begin to form at their outer ends 

 a layer of ivory (Fig. 971, IV.). Similarly, the 

 adamant cells lining the cup begin to form at their 

 inner surface a layer of adamant on the top of the 



layer of ivory (Fig. 971, IV.), to which it adheres: Fm - 970.-DIAGKAM TO ILLUSTRATE THE 

 in each case the deposit taking place first at the sum- 

 mit of the tooth. 



5. The formation of these tissues proceeds apace, 

 the ivory increasing at the expense of the papilla, 

 the adamant similarly encroaching on the cup or 



! adamant organ ; and in each case the two layers of cells odontoblasts and adamant cells 

 which produced the deposits, retiring gradually from one another, as the space between 

 them becomes occupied by the newly formed tissues (Fig. 971, V). 



The remains of the dental papilla persist as the pulp of the tooth, which is covered 

 even in the adult by the odontoblasts, and occupies the tooth cavity, i.e. the central part 



: of the tooth to which the formation of ivory has not extended. 



6. Turning now to the jaw itself : The connective tissue of the gum surrounding the 

 tooth germ (as the developing tooth with its adamant organ and dental papilla are called) 

 early becomes condensed and vascular (Fig. 971, V.), and later on forms a membranous 

 bag the tooth-sac or follicle which completely shuts off the developing tooth from the 

 surrounding structures. On the floor of the sac the tooth germ sits, the base of its 



. dental papilla being continuous with the tissue of the floor of the sac, and the young 

 tooth being enclosed by the sac, as a kernel is enclosed by its shell. 



7. Reverting to the tooth : When the crown is completed the deposit of ivory, but 

 not of adamant, is continued downwards to form the root. The root is composed chiefly 

 of ivory, continuous above with that of the crown, and like it formed by the odontoblasts 

 of the dental papilla. As the ivory is deposited, and the root is being built up, the 

 connective tissue of the tooth-sac comes to surround the root more closely, and deposits 

 on its surface, after the manner of a periosteum, a layer of bone, the substantia ossea. 

 After that layer has been formed, the connective tissue of the sac persists as the alveolar 

 periosteum. The development of the root takes place very slowly, and its lower end is 

 not completed as a rule for some time after the eruption of the tooth has taken place. 



8. During the development of the teeth the ossification of the jaw has been going on, 

 and as it grows up on each side, the young teeth, enclosed in their tooth-sacs, come to lie 

 in an open bony groove, which is subsequently divided by septa into compartments the 

 alveoli for the individual tooth-sacs. The bone continuing to grow after birth, these 



A, Section of skin 

 showing epithelium 

 e, basement mem- 

 brane ft, and connec- 

 tive tissue layer c. 



B shows the papilla of 

 the connective tissue 

 layer growing up 

 covered by the epi- 

 thelial layer. 



In C the superficial cells 

 of the papilla z begin 

 to deposit ivory d 

 over the papilla, and 

 at the same time the 

 deepest cells of the 

 epithelium deposit 

 adamant a. 



D shows the tooth 

 breaking through 

 the epithelium and 

 reaching the surface. 



DEVELOPMENT OF A DERMAL TOOTH IN 

 THE SHARK. 



In all figures a, adamant ; &, basement mem- 

 brane ; c, connective tissue layer of skin ; 

 d, ivory ; e, epithelium ; and z, superficial 

 cells of papilla. 



