62 Upon the Development of the Enamel in certain Osseous Fish. 



in the formative processes. The present communication seeks to 

 establish an additional method of enamel formation, essentially differing 

 from any which has hitherto been described, and whilst the investiga- 

 tion was undertaken in the hope of bridging over the gaps which exist 

 between the methods previously known, it has only partly succeeded in 

 doing so, as the process to be described stands somewhat alone. 



The principal point which is set forth in the paper is, that the forma- 

 tive cells of enamel, known as ameloblasts, in all the Gadidse and in 

 Sargus and Labrus, undergo a preliminary transformation into a 

 reticulated stroma, which is of the full dimensions of the ultimate 

 enamel. During the calcification of enamel, the ameloblasts no longer 

 exist as such, nor do any other cells take their place, but the stroma 

 itself seems able to segregate and properly apply the lime salts re- 

 quired, which make their appearance at that side of the thick stroma 

 which is most distant from the blood vessels. 



There have thus been demonstrated four varieties of the process by 

 which enamel is formed, which, although there are gaps not at present 

 bridged over, may perhaps be taken as representing certain stages in 

 the evolution of enamel. These are — 



1. Enamels not wholly epiblastic in origin, in which the stroma 

 which is the seat of enamel calcification is furnished by a transforma- 

 tion of the exterior of the mesoblastic dentine papilla, the ameloblasts 

 apparently segregating the lime salts required for its hardening. This 

 is found in the Elasmobranch fishes. 



2. Enamels wholly epiblastic in origin, in which the ameloblasts 

 undergo a prior transformation into a stroma of the dimensions of the 

 finished enamel, and themselves disappear. This is the subject of the 

 present communication, and is met with in the Gadidse, in Sargus and 

 in Labrus, and probably in many other fish. 



3. Enamels wholly epiblastic in origin, in which the ameloblasts 

 retain their integrity throughout the whole process. Their extremi- 

 ties are, however, produced into long fibrillar processes, which are 

 traceable far through the calcifying enamel, and these processes are 

 prolongations of the plasm of the cells. This method is found in the 

 enamel of Marsupials, and probably in all similar tubular enamels, 

 such as are found in Hyrax and sporadically among other mammals. 



4. Enamels wholly epiblastic in origin, in which the ameloblasts 

 persist throughout the process of calcification. Their free ends are 

 produced into short processes (Tomes's processes), which penetrate but 

 a short distance into the calcifying enamel. This is the ordinary 

 method found in placental mammals. 



It will be seen that the last two methods difi'er in degree rather than 

 in kind, but that the first two stand markedly apart. 



Apparently in the Eays there is some sign of the approaching aban- 

 donment of the share taken by the dentine papilla, as there is a less 



