310 



EMBRYOLOGY. 



the cells assume spindle-shaped forms and group themselves into a 

 kind of epithelial layer, the layer of the dentine-forming cells (mem- 

 brana eboris). On the part of the cap-like enamel-organ the cells of 

 the deepest layer, which is in immediate contact with the papilla, are 

 converted into very long cylinders and constitute the enamel-mem- 

 brane (sm, membrana adamantine). The latter becomes gradually 

 thinner toward the base of the papilla, where it is continued as a 

 layer of more cubical elements (se), which forms the boundary at the 

 surface of the cap separating it from the surrounding connective 

 tissue. Between these two cell-layers (the inner and the outer 

 epithelium of KOLLIKER) the remaining epithelial cells of the enamel- 

 organ undergo a peculiar metamorphosis, and produce a kind of 

 gelatinous tissue, the enamel-pulp (sp) ; they secrete between them a 



A _> 



Fig. 172 A B. Two stages in the development of the teeth of Mammals. Diagrammatic sections. 



zj\ Dental groove ; zl, dental ridge ; zl 1 , deepest part of the dental ridge, on which are formed 

 the fundaments of the supplementary teeth ; zp, dental papilla ; am, enamel-membrane ; 

 .sy>, enamel-pulp ; se, outer epithelium of the enamel-organ ; zs, dental sac ; k, bony alveolus. 



fluid rich in mucus and albumen, and become themselves converted 

 into stellate cells, which are united to one another by their processes, 

 and thus form a fine network. The enamel-pulp is most highly 

 developed in the fifth or sixth month, and then diminishes up to the 

 time of birth in the same ratio as the teeth increase in size. 



The connective tissue immediately enveloping the whole fundament 

 acquires numerous blood-vessels, from which branches also make their 

 way into the papilla ; it becomes somewhat differentiated from the 

 surrounding tissue, and is distinguished as dental sac (fig. 172 B zs). 



The soft fundaments of the teeth enlarge up to the fifth month of 

 embryonic life, and at the same time acquire the particular forms of 

 the teeth which are to arise from them those of the incisors, the 

 canines, and molars. Then the process of ossification begins (fig. 173) 

 in the same manner as in the dermal teeth. A cap of dentine (zb) is 



