DEVELOPMENT OF THE ENAMEL 193 



development of the enamel in osseous fish has been but very imperfectly 

 studied, and it is only by examining a large series of specimens of the 

 various families that any systematically arranged conclusions can be 

 arrived at ; but that very remarkable variations in the structure and 

 arrangement of the enamel organ do exist we appear to have very complete 

 evidence. The remarkable fact that the circulating blood is brought 

 into intimate relations with the cells of the enamel organ within its sub- 

 stance certainly points to a mode of development of the enamel which seems 

 to have no parallel in other vertebrates. 



In the Elasmobranchii, according to Tomes, both the Summary, 

 epiblastic epithelial cells and those of the mesodermic 

 dentine papilla take part in the formation of the enamel, 

 the enamel being calcified by the ameloblast cells in a matrix 

 which is furnished by the dentine papilla. In the osseous 

 fish the investigation of only a few families has been 

 thoroughly carried out, but while in many the enamel organ 

 is both in structure and function similar to that of the 

 higher vertebrata, in those which possess a more or less 

 tubular enamel a very different structure of the enamel 

 organ is found. Although at first, as in the Gadidae, in 

 the Sparidae and Labridae the enamel organ is composed of 

 a true layer of ameloblasts and an external epithelium, only 

 a very narrow zone of enamel is laid down by the amelo- 

 blasts, which soon disappear, a delicate stroma taking their 

 place, and within this stroma is differentiated an enamel 

 organ of an altogether different structure, consisting in the 

 case of Sargus and Labrus of an apparently definite secreting 

 apparatus, the lime salts being elaborated by cells which 

 are arranged between and around vascular tubes which 

 penetrate the enamel organ from the capsule. The calcifying 

 substance is conveyed by special channels to the stroma, 

 which occupies the interval between this structure and the 

 forming enamel and in which calcification takes place. In 

 Tautoga the secreting structure takes the form of tubular gland- 

 like structures regularly disposed within the enamel organ. 



In the Gadidae no such definite tubular structure has been 

 described, but C. S. Tomes was the first to draw attention 

 to these changes in the enamel organ. No blood-vessels 

 are present in the enamel organ of the Gadidae, and according 

 to Tomes there are no cells to which can be assigned the 

 function of separating the lime salts. 



