112 T. CHARTERS WHITE ON THE 



at the commencement of the alimentary canal, and only a few 

 inches from the external surface of the body, yet in the lower 

 animals, they may be found occupying a great range of positions, 

 either studding the formidable rostrum of the saw-fish, and de- 

 veloped into the mighty spear of the narwhal, or found in tlie 

 digestive cavity of the Crustacea, and being periodically shed with 

 the shells of these creatures. 



These few instances may be quoted as proofs of their true rela- 

 tion to the external skeleton, proofs wbich I hope may be made 

 still further clear, when we come to deal with their developmental 

 history. 



With these few words of introduction, we will proceed to an 

 examination of these structures in the earliest periods of their 

 existence. 



The development of the teeth commences about the sixth week 

 of foetal life, by the formation of a ridge upon the gums of the 

 upper and lower jaws, and may be studied most advantageously in 

 sections taken during this and the succeeding periods of growth, 

 when may be observed a thickening of the embryonic connective 

 tissue and epithelium of the mucous membrane of the mouth. 

 This epithelium, with its vascular substratum, constitutes, there- 

 fore, the matrix of the several constituents of the future tooth, the 

 epithelium forming the enamel, and the mucous tissue the dentine 

 and cement, while the surrounding tissues immediately investing 

 the rudiments of the future tooth become a sacculus enclosing it 

 during the changes which attend its formation till it is ready to be 

 evolved, or in popular parlance, till it is ' cut." By a reference to 

 these diagrams, taken from Kolliker, this process may be more 

 readily understood. Near the end of the second month of fcetal 

 life, a longitudinal furrow, with rounded borders, makes its appear- 

 ance on the gums ; the epithelium of the oral cavity completely 

 covers it, so that it is only perceptible with difficulty if the surface 

 alone be examined, but in a section of the foetal lower jaw, from 

 which the diagram is taken, a dipping down of the deepest portion 

 of the epithelial layer into the subjacent mucous tissue may be 

 observed. This depression, narrow at first, ultimately becomes 

 wider at its lower end, and is the agent concerned in the forma- 

 tion of the enamel organ, as we shall see as development takes 

 place. 



A series of remarkable changes now occur in the more deeply- 



