DEVELOPMENT OF THE TEETH OF FISHES. 
261 
that they may almost be termed a “ compound enamel-organ; ” and they thus recall the 
manner in which successive enamel-organs bud off from the necks of their predecessors 
in the newt. 
The columnar cells (enamel-cells) remain upon the surface of the formed tooth until 
it has arrived at a point above the protection of the thecal fold (see the third tooth in 
fig. 1) ; after this they get worn off and lost. 
In the interspaces of the teeth the “ enamel-organs ” lose their distinctive character, 
and between the older teeth do not markedly differ from the epithelium of other parts 
of the mouth. 
In young dogfishes the partitions between the successional teeth, and also between 
the contiguous vertical rows, are formed solely by epithelium ; but, as was pointed out 
by Professor Huxley, in older specimens septa of connective tissue separate each ver- 
tical row from those on either side of it, so that a tooth and its successors are contained 
in a sort of longitudinal canal, the whole series being connected together by the conti- 
nuity of their enamel-organs. 
In very young specimens, before any lip is formed, the transition of the dermal 
spines on the under surface of the head into the teeth is readily demonstrable. 
Thus in fig. 3, which represents a section of the lower jaw of a very young dogfish, 
the spines upon the skin are seen to pass without breach of continuity into the teeth, 
from which they differ mainly in size, and to a very slight extent only in shape *. 
It is stated by Gegekbauk (Manuel d’Anatomie Comparee, p. 738) that in Selacliia 
the mucous membrane of the mouth, as far back as the pharynx, is clothed with spines 
of structure identical with that of the teeth proper, these spines often occupying those 
regions which in Ganoids and Teleostei are clothed with conspicuous teeth ; I have not 
had the opportunity of myself verifying his statement f. 
* Prof. 'Williamson (Phil. Trans. 1849) pointed out the structural resemblance of ganoid and placoid sides 
to teeth and their homological identity. 
t To the above remarks must he added the statement of Heetwig (Jenaische Zeitschrift, 1874), that the 
teeth and the dermal spines are developed in a manner precisely similar. I did not become acquainted with 
his paper until after this paper had been read before the Loyal Society, or I should have made more frequent refer- 
ence to his results. Dr. Heetwig seems to have been working in the same groove as myself, and has published 
his paper relating to the development of the dermal spines and teeth of sharks a short time before mine was 
read ; whilst a short time after my paper on the development of Batrachian teeth was read he published a con- 
tribution to the same subject. It is gratifying to find that the figures and descriptions of our independent and 
almost contemporaneous papers conform very closely, though there are points on which we differ. 
Dr. Heetwig gives figures of dermal spines in the process of development in which a papilliform eminence 
of the dermis is undergoing calcification at its tip, whilst the stratum Malpighii over it is furnishing a crust of 
enamel, neither the “ enamel-organ” nor the “ dentine-papilla,” if such they can he called, being markedly 
specialized nor different from the parts around them, save in contour and in the size of the epithelial cells. 
He holds that the increased size &c. of the dermal spines over the surface of the jaws, where they constitute 
teeth, is adequately accounted for on the principle of increased use ; that is to say, the excitation of pressure 
will bring about increased vascularity, and so forth. 
His figures are very clear and accurate ; and he has entered into the details of the process of calcification with 
