DEVELOPMENT OF THE ENAMEL 145 



the ameloblasts are receding as calcification advances, they 

 do not themselves undergo conversion into enamel, but their 

 processes, the Tomes' processes to be afterwards described, do 

 become incorporated in it and form the organic foundation of 

 the finished tissue. This will be more clearly understood when 

 the appearances seen in forming enamel have been described. 



While in Mammalia the ameloblast cells are undoubtedly 

 the formative agents of the enamel, it has been pointed 

 out by C. S. Tomes in the Gadidse (26 b) among osseous 

 fish, and in Sargus and Labrus by the author, that although 

 in the earliest condition of the tooth-germ in these fish the 

 ameloblasts are distinctly present, they disappear at an 

 early stage, and their place is taken by other structures, into 

 which the enamel cells have been apparently converted. 



This disappearance of the ameloblast cells, as such, is 

 especially remarkable, as even in the placoid scales of 

 Elasmobranchs (the hard prominent tubercles of the skin of 

 the sharks and rays) a layer of ameloblasts is distinctly 

 evident during their development, and enamel appears to 

 be formed by their agency, as in the true teeth. This will 

 be further referred to in considering the development of 

 the enamel in fish. 



In the study of the calcification of mammalian enamel 

 we shall include that of the tubular enamel of marsupials, 

 for although the higher existing Mammalia may not be in 

 the direct line of descent from the marsupials, the structure 

 of their enamel is in all essential particulars similar to that 

 of human teeth, and the presence of tubes probably only 

 an indication of less complete calcification. The enamel is 

 formed in the same manner as in higher orders, and is, as 

 it were, a stage in the development of the teeth of more 

 advanced forms. It is more slowly and less completely 

 calcified, and consequently the stages in its formation are 

 more easily studied. 



A tubular condition of the enamel is seen in Hyrax, Jerboa, 

 and the Shrews, where the enamel is in other respects similar 

 to the enamel of other orders of the higher mammalia. 



It may be stated that although in man and the higher 

 mammalia no blood-vessels are seen deeply within the 

 enamel organ, it was shown by Malassez, as explained 



MUMMERY 



