130 IGERNA B. J. SOLLAS. 



are agreed that the odontoblasts give origin to the teeth^ but 

 some would have it that the roofing epithelium secretes a kind 

 of enamel or hard outer layer which is spread over the 

 surface of the tooth originally laid down by the odontoblast. 

 Among those wlio take this view areRossler (15), Riicker (14), 

 Sharp (18), and Bloch (5). BIocli describes the enamel layer 

 as consisting of specially hard cuticular substance. Two 

 writers, more recently — Rottmann and Schnabel — deny the 

 presence of any such enamel, and maintain that the teeth are 

 laid down from the first in their definitive form and size, and 

 that the roofing epithelium contributes no substance whatever 

 to the radula, and is not to be regarded as secretory. There 

 is further difference of opinion as to the exact method by 

 which the forward movement is brought about, and on other 

 points. 



The facts which have been already stated in dealing with 

 the Docoglossan and other radulge place beyond all doubt the 

 importance of the roofing cells of the radular sac in all 

 cases; in the case of Patella the lateral teeth as first 

 formed are soft and colourless, and it is only those situated at 

 a considerable distance from the odontoblasts which betray 

 by their yellow colour the presence of iron oxide, and this 

 must have come from the roofing cells. The other changes in 

 the maturing teeth all point to the secretory nature of the 

 roofing epithelium, a function which is strongly suggested 

 a priori by the conspicuous accuracy with which the cells 

 of this epithelium fit in between the teeth, leaving not the 

 minutest portion of tooth or membrane surface untouched, 

 Schnabel and Rottmann were working with material other 

 than Docoglossa, namely with Gastropoda and Cephalopoda. 

 Therefore it will be worth while to consider at any rate one 

 of these cases in greater detail. That some considerable 

 changes do take place in the teeth after their first formation 

 is already clear, but the changes as seen in microscopic 

 sections are interesting and afford some further light. 



The young teeth, as we know from dissection are soft ; they 

 take protoplasmic stains but slightly, and agree in this with the 



