194 MICROSCOPIC ANATOMY OF THE TEETH 



Further investigation is necessary to determine if this 

 conversion of the first-formed enamel organ into a secretory 

 structure is peculiar to the enamels which contain tubes 

 from the outer surface, but the fact that the first-formed 

 layer of enamel in all the fish examined contains no tubes 

 and is laid down under the influence of normal ameloblasts. 

 would tend to indicate that the various modifications of 

 this secretory structure are peculiar to those forms which 

 possess tubular enamel. 



From the evolutionary standpoint the method of calcifica- 

 tion above described presents a curious problem. In 

 Elasmobranchs we see the first evidence of the formation 

 of enamel, which takes place by the agency of true amelo- 

 blasts, arranged as a well-defined internal epithelium of the 

 enamel organ as in mammalia, and it is also present in the 

 placoid scales of the skin of Elasmobranchs. While this mode 

 of calcification of the enamel appears to be exactly similar 

 to that of the higher vertebrata, in many Sparidse and 

 Labridse, although the deposit of enamel begins in this 

 manner, only a very small amount of the tissue is deposited 

 under these conditions, and the secreting structure before 

 described takes up the functions of the ameloblasts. 



The attention of the author was drawn, by J. T. Carter 

 to a paper by N. R. Harrington (11), in which he describes 

 the microscopic structure of the lime- secreting glands of the 

 earthworm (Lumbricus). In these glands there is a regular 

 arrangement of sinuses or vascular tubes with secreting 

 cells between them, but there are no separate definite 

 channels for the conveyance of the secretion as in the fish, 

 but it passes back between the laminae to be discharged into 

 the oesophagus. The author has been unable to trace any 

 other structure of a similar nature adapted to this purpose, 

 either among the vertebrata or in vertebrata. 



A comparison might also be made with one of the ductless 

 glands as far as structure is concerned, although it is not 

 engaged in the secretion of lime salts. Dahlgren and 

 Kepner (7) figure a section through the infundibular gland 

 of the flounder (Pseudopleuronectes americanus) . This shows 

 that the secreting cells are arranged along sinusoids filled 

 with blood ; each sac -like invagination of the secreting cells is 



