MUSEUM OF COMPARATIVE ZOOLOGY. 123 



Almost as soon as the calcai'eous material of tlic scale begins to be 

 laid down some of tlio surrounding scleroblasts become enclosed by it 

 (Fig. 3), This process goes on hand in hand with the increase in the 

 size of the scale, and as a result the scleroblasts are distributed tlirough 

 all parts of it. That the distribution is a fairly r(>gidar one may be 

 seen from Figure 15 (Plate II.) and Figure 21 (Plate III.). In a scale 

 from which the soft parts have been removed by treatment with caustic 

 potash, the cavities occupied by these cells may be very distinctly seen. 

 Each one has leading from it a small number of canals (caualiculi) 

 which branch and traverse the scale to unite with similar canaliculi 

 from the neighboring cavities. Thus the whole scale is traversed by a 

 network of fine ramifying tubules connecting the osteoblastic cavities. 

 As may be seen in section, however, all the canaliculi from a given 

 cavity show a tendency to spread out in a plane parallel to that surface 

 of the scale within which it has been buried, so that the cavities in most 

 intimate connection are those which lie at the same distance below the 

 surface. Thus the material of the scale is divided into more or less 

 regular lamellte of calcareous matter alternating with successive layers 

 of cell cavities and their connecting canaliculi. These cavities are in 

 communication with the exterior by means of canals which penetrate 

 the scale and break up at their inner ends into fine tubules to join the 

 canaliculi. They penetrate from both the upper and the lower surface, 

 though much more abundantly from the lower (Plate II. Fig. 1.5, and 

 Plate III. Fig. 21). At the opening of each of these canals at the 

 surface of the scale there is a large cell (Plate I. Figs. 8 and 9, Plate III. 

 Fig. 22, Plate IV. Fig. 23), from which a process extends into the lumen. 

 These cells were called by Hertwig " Odontoblasten," and the canals oc- 

 cupied by their processes " Dentinrohrchen." I must, however, agree with 

 Klaatsch that the names are poorly chosen, for the reasons set forth by 

 him. The constant character of dentine, as the term has been used, 

 is the absence of enclosed cells, and as the substance penetrated by 

 the processes of these cells contains such elements (osteoblasts), it seems 

 undesirable to call it dentine or the cells odontoblasts. Hertwig's nomen- 

 clature rests on an assumption of homology which has not been proved 

 true. However, as these terms have been adopted in the literature, it is 

 perhaps inadvisable to introduce new ones at this time. Besides the 

 odontoblasts with their processes extending into the dentinal tubules 

 which Hertwig describes and figures ('79, p. 5, Taf. III. Fig. 4), he 

 mentions the presence of a granular substance partially filling the lumen 

 of the canals. Klaatsch asserts that cells are present in the dentinal 



