260 
ME. CHAELES S. TOMES ON THE 
tom at this point, as often happens, and the continuity is therefore interrupted at f in 
the figure. 
All that lies below this point is situated, therefore, not in an open groove, hut in a 
closed space, roofed in and filled in by epithelium ; the significance of this fact only be- 
comes fully apparent when taken in conjunction with what is observed in other creatures. 
Below this point the epithelium is continued down in the interspace between the jaw 
and the thecal fold, and forms investing caps to each of the dentine-papillae — attaining, 
opposite to the youngest of the latter, a degree of development apparently in advance 
of and almost disproportionate to the stage of evolution of the papilla. 
Their homologies, and the degree of development which they attain, entitle these 
epithelial caps to the name of “enamel-organs”*; and it was first pointed out by Pro- 
fessor Huxley, in the paper to which I have already referred, that enamel-organs are 
“nothing more than altered epithelium ” f. 
The structure of these enamel-organs is not very dissimilar to that of the correspond- 
ing organs in other Fish and Reptiles ; their most conspicuous part is a layer of regular 
large columnar cells (“enamel-cells”), furnished with nuclei at their attached ends, and 
measuring from -5^0 to -foo an inch in length. These “ enamel-cellsj” are larger than, 
and different in appearance from, the cells to he met with elsewhere in the epithelium 
of the mouth ; their occurrence, as well as the character of the residuum of the enamel- 
organ, justify us in saying that the epithelium “has in this situation undergone a 
special transformation into an 4 enamel-organ.’ ” 
Unless the specimen has been treated with acid the enamel-cells are often found to 
be firmly adherent to the cap of forming dentine ; hut after prolonged maceration in 
chromic acid they become detached, as is the case in the sections figured. 
The extent to which enamel is formed upon the teeth of Elasmobranch Fishes is vari- 
able : it may seem an easy matter to pronounce whether there is or is not enamel upon 
any particular tooth ; but in actual practice it is a matter of extreme difficulty to pro- 
nounce with any thing approaching to certainty upon the presence of an exceedingly 
thin layer of enamel upon the exterior of a tooth. 
No one can doubt the existence of enamel upon the teeth of some skates ; as the for- 
mative organs of the teeth of the skates are precisely similar, save in some details of 
form, to those of the dogfish, I do not doubt that the thin clear external layers upon 
the teeth of the latter are to be regarded as enamel. 
The residual part of the enamel-organ is inconspicuous ; it consists of very much 
smaller cells, branched and intercommunicating and forming a firmly fibrillated tissue ; 
it is best seen in the youngest enamel-organ (fig. 2). 
The enamel-organs of the successional teeth are so closely and intimately connected 
* Of. the “ enamel-organs ” of the dermal spines of dogfish, as described and figured by Hertwig, Jenaische 
Zeitschrift, 1874. 
f Prof. Kolliker (Mikros. Anatomie, B. ii. p. 114) by inference denies the existence of an enamel-organ 
in the Plagiostomes ; I am not aware of his having since altered his opinion. 
