THE DEVELOPMENT OF THE TEETH. 157 



As has been already mentioned, Kolliker dissents from the above 

 account of the calcification of. the enamel, partly on the ground 

 that enamel cells may be seen of the same size and form at all 

 stages of the formation of enamel. 



The process he regards as one of secretion, the enamel being shed 

 out, so to speak, from the free end of each enamel cell ; hence the 

 prisms of the enamel will correspond in size and number with the 

 cells of the enamel epithelium ; the processes of the enamel cells 

 he regards as being fragments of this hardened secretion which are 

 Btill clinging to the parent cell. 



M. Magitot (Journal de 1'anatomie de M. Ch. Robin, 1879) has 

 revived this view, describing each cell as terminated, towards the 

 forming enamel, by a little plate of dense material through which 

 by some process of exosmosis the constituents of enamel travel out. 

 He notes that these plates often cohere so as to form a sheet (cf . 

 page 154), but says nothing of their being perforated. No one, 

 however, who had seen the enamel cell of a marsupial with the 

 tapering process five or six times as long as itself which had been 

 pulled out of the young enamel would be satisfied with the excre- 

 tion theory. 



The reasons for adopting the opposite view will have been ga- 

 thered from the text ; they are, in brief, the occurrence of the 

 11 Tomes' processes," especially in marsupials ; the rigidity of the 

 open mouths of the enamel cells ; the pitted surface of the youngest 

 layer of enamel, the foraminated membrane which can be raised 

 from it, and the relation of these facts to the occurrence of the 

 processes of the enamel cells. 



Schwann believed that the enamel cell was constantly increasing 

 at its free end (.?., that next to the enamel), and that the new 

 growth, or youngest part of the cell, is calcified as fast as it is 

 formed ; this view differs little from that of Kolliker, who prefers 

 to express it by saying that this end of the cell is constantly shed- 

 ding off or secreting a material which becomes external to itself. 

 My father, Waldeyer, Hertz, and many others, believe that the cell 

 growth takes place not at this free end, but at the attached nucle- 

 ated end, and that it is the oldest portion of the cell itself which 

 receives an impregnation with salts and forms the enamel. 



Professor Huxley's opinion (page 152) is, I take it, based on the 

 fact that a membrane could be raised from the surface of young 

 enamel, which must have intervened between the enamel cells and 

 the enamel prisms ; if my father's explanation of the nature of this 

 membrane be accepted, the difficulty vanishes. 



My own researches upon the development of the teeth of fishes 

 also furnish evidence ten ding in the same direction ; as has been al- 

 ready mentioned, the enamel cells in some parts of the enamel organs 

 of certain fish, such as the eel and perch, and certain Batrachia, 

 e.g., the newt, have dimensions very greatly exceeding those of the 

 cells in the remainder of the organ. These highly developed cells, 



