40 The Microscope, 



kind of cells cannot produce another kind, we recognize the 

 early manifestations of that natural law which keeps them 

 separate, yet wonderfully harmonizes and adapts each to the 

 other in one product, the tooth. After having briefly. con- 

 trasted the origin and development of these tissues, we will 

 conclude by answering the question asked at first, by saying that 

 in the formation of most tissues provision is made for repair. 

 In bone by the encapsulated osteoblasts ready when called upon 

 to secrete, as in the development, calcareous matter ; while in 

 the enamel the source of the supply of material being lost in 

 the disappearance of the ameloblasts and rupture of the tooth 

 through the gum, no provision is made by Nature for repair 

 should this interesting structure be destroyed. 



EXPLANATION OF FIGURES. PLATE I. 



Fig. I. — X 50. Section through the parietal bone of a two months 

 human embryo, showing the osteoblasts o, and ealcospherules, p. 



Fig. II.— X 750. Horizontal section at the junction of a rib and trans- 

 verse process of a dorsal vertebra in a two months iiuman em- 

 bryo ; g, osteoblasts ; h, ealcospherules. 



Fig. III. — X 100. Transverse section through the inferior maxilla of a 

 six weeks human embryo; w, epithelium; x, band for future 

 tooth ; y, infant layer; z, Meckel's cartilage; v, developing bone. 



Fig. IV. — X 300. Transverse section through the inferior maxilla of a two 

 months human ftetus; a, infant layer, epiblasts accidentally sep- 

 arated from the mesoblasts; b, epithelium ; c, band; of, cord for 

 tooth ; e, connective tissue mesoblasts ; /, Meckel's cartilao-e. 



Fig. V. — X 65. Transverse section through the inferior maxilla of a three 

 months human foetus; k, epithelium ; m, neck of cord ; n, infant 

 layer, o, outer tunic ; p, inner tunic ; q, dental papilla ; r, develop- 

 ing bone ; s, Meckel's cartilage. 



Fig. VI. — X 125. Transverse section through the inferior maxilla in the 

 region of a cuspid of a 4 months human foetus ; a, epithelium ; 

 I), infant layer; c, cord for permanent tooth ; d, outer tunic; e, 

 inner tunic ; / dental papilla ; g, odontoblasts ; h. ameloblasts : 

 j, stellate reticulum. 



Fig. VII. — X 65. Transverse section through the inferior maxilla, region 

 of a molar, of a young mouse two or three days old ; /, epi- 

 thelium ; n, outer tunic; o, ameloblasts; p, dental papilla; s, 

 odontoblasts; t, hair follicle. 



Fig. VIII. — X 1000. Enlargement from Fig. VII at the point m ; a, 

 ameloblasts ; b, stratum intermedium of Hanover ; t, Tomes' 

 processes ; e, enamel newly formed ; d, dentine ; o, odontoblasts ; 

 s, dentinal pai)ilia. 



Fig. IX. — X 350. Section througli the tip of a human incisor; ;>•, enamel 

 prisms ; f, dentine. The section was ground as thin as possible 

 and afterward treated with dilute hydrochloric acid, 1 to 500, to 

 separate the prisms. 



