DEVELOPMENT OF THE SCALES ilND BONES OF FISHES. 
671 
share to the formation of the small oval interspaces seen in fig. 29. The surface of 
the section is seen to be crowded over with small round or oval cavities in the solid 
calcareous substance. These cavities are frequently linked together by narrow necks 
or irregular tubular canals, and in the projecting portions exhibit a strong tendency 
to assume a linear arrangement in groups which radiate from the centre, those of 
each group being more or less parallel to one another. In the calcareous inter- 
areolar substance, numerous concentric lines of growth’are faintly visible, especially 
traversing the projecting processes at right angles to their direction. 
Fig. 31 is a vertical section of one of these osseous plates, with a portion of the 
subjacent cartilage. At 31 awe perceive that the cartilage-cells present the ordinary 
ichthyal form, being gathered together in small detached groups. As we approach 
the inner surface of the osseous plate these groups begin to break up, and the cells 
re-arrange themselves in divergent lines (31 h), radiating from the centre of the cal 
careous structure. At the same time that their distribution is thus changed, each 
cell becomes more turgid, and exhibits more definite margins than before. The edges 
of the calcareous plate when thus intersected are seen to be thin, whilst its centre 
projects considerably into the cartilage; it presents the same array of internal cells 
that were seen in the horizontal section, fig. 30, but here their direction is changed, 
being nearly the same as that of the contiguous cartilage-cells. A careful examina- 
tion of the line 31c, where the two substances are in contact, makes it very evident 
that the calcareous matter is deposited, in the form of minute granules, in the inter- 
cellular spaces, which granules resemble, in their more essential features, those form- 
ing the calcareous laminae of Cycloid scales, only being round instead of lenticular. 
They obviously increase in size by the addition of concentric layers, and by the 
coalescence of previously isolated granules. The small cavities correspond with 
what were originally cartilage-cells, but which, not having been filled up with the 
calcareous matter, have remained as small circular cavities in the solid structure. 
The point 31 d has evidently been the centre of ossification, and the new additions 
have been made both to the inner surface and to the peripheral margins of the plate ; 
whilst the cavities in the centre of the structure have been arranged more or less 
vertically, the linear rows nearer the margin have been so deflected as to be almost 
parallel with the outer surface of the ossifying cartilage. This correspondence be- 
tween the direction of the cavities and those of the contiguous cartilage-cells, which 
is important, as establishing the connection that exists between them, is also seen 
on examining one of the small circular interosseous points of cartilage visible on the 
surface, 29 d. Fig. 32 represents one of these cavities, into the composition of which 
portions of three osseous plates have entered (32 a). The cartilage-cells are here 
arranged in three arched series, and the cartilage itself assumes a semi-fibrous aspect, 
the fibres running in the same direction as the cells, which again corresponds with 
that of the small cavities in the contiguous processes of the calcareous plates. 
When one of these plates is decalcified, the animal basis retains the contour of the 
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