66 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



Each papilla early takes the shape of the crown of the tooth to which 

 it corresponds, but as the dentine increases in thickness the papilla diminishes 

 until when the tooth is cut only a small amount remains as the pulp. It is 

 supplied by vessels and nerves which enter at the end of the root. The roots 

 are not completely formed at the time of the eruption of the teeth. 



FIG. 67. Vertical Transverse Section of the Dental Sac, Pulp, etc., of a Kitten, a, Dental 

 papilla or pulp; b, the cap of dentine formed upon the summit; c, its covering of enamel; d, inner 

 layer of epithelium of the enamel organ; e, gelatinous tissue; /, outer epithelial layer of the enamel 

 organ; g, inner layer, and h, outer layer of dental sac. X 14- (Thiersch.) 



The enamel cap is formed by the enamel cells, by the deposit of a keratin- 

 like substance, which subsequently undergoes calcification. Other layers 

 are formed in the same manner meanwhile. 



The temporary or milk-teeth are speedily replaced by the growth of the 

 permanent teeth. 



The development of the temporary teeth commences about the sixth 

 week of intra-uterine life, after the laying down of the bony structure of 

 the jaws. Their permanent successors begin to form about the sixteenth 

 week of intra-uterine life. 



III. MUSCULAR TISSUE. 



There are two chief kinds of muscular tissue, differing both in minute 

 structure as well as in mode of action, viz., (i) the smooth or non-striated, and 

 (2) the striated. 



