132 BULLETIN OF THE 



comparable to it with whicli I am acquainted is that of cartilage cells, 

 which in the formation of endochondral bone become transformed into 

 osteoblasts; i. e. the character of the secretion of the same cells is 

 different in the different periods of their activity, and hence to that 

 extent the two cases are similar. 



In young scales in which the ganoin layer is still thin it forms an 

 even coat, and the striation visible in it is parallel to the surface. In 

 sections of older scales there are, near the edges, a series of notches in 

 the lower surface of the ganoin which conform to inequalities of the 

 upper surface of the underlying bony layers (Plate II. Fig. 16). Similar 

 conditions have already been described and figured by Williamson and 

 others. Figures 15 and IC (Plate II.) show that each notch marks the 

 point which was once the edge of the ganoin layer, and that the forma- 

 tion of ganoin on one side of this point (right. Fig. 16) and of bony 

 material on the other must have gone on for some time without any 

 lateral extension of the ganoin taking place. It is also evident that 

 extensions of this layer equivalent in amount to the distance between 

 successive notches must have taken place periodically, not by contin- 

 uous growth. The cause of such periodicity I have no means for 

 determining. 



The fine striations (lamellation) in the ganoin which have already 

 been mentioned have directions in this part of the layer slightly different 

 from those in the region farther from the margin of the scale. Instead 

 of being parallel with the upper surface, they are conformable to the 

 earlier surfaces of the layei", and so have a dip downward toward the 

 underlying bony material as they approach the edge of the scale. They 

 also gradually diminish in thickness toward the central area of the 

 scale, showing that the process of secretion went on less rapidly there 

 than it did nearer the margin of the scale. 



The " tubes lepidines " of Williamson are clearly visible in ground 

 sections, though not present in cut sections of decalcified scales. They 

 are due to the presence of uncalcified connective-tissue fibres, as has 

 been stated by Klaatsch. These fibres in drying shrink, and so leave 

 minute spaces about them which the balsam does not enter (thl. Ipd., 

 Plate 11. Fig. 15, and Plate III. Fig. 21). The course of these "tubes" 

 is very characteristic, and is shown in Figure 15. They are absent in 

 the part of the scale immediately beneath the ganoin layer ; they begin 

 in that part which was first formed, and from here they radiate, — the 

 directions being downward in the middle of the scale and diagonally 

 downward and outward near either end of the section. 



