182 BULLETIN OF THE 
comparable to it with which I am acquainted is that of cartilage cells, 
which in the formation of endochondral bone become transformed into 
osteoblasts; i. c. 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 16 (Plate IL.) 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 
beon 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 
carlier surfaces of the layer, 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 (tbl. Ipd., 
Plate II. Fig. 15, and Plate IIT. 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 cither end of the section. 
