STRUCTURE AND GROWTH
be viewed either as canaliculi which penetrate from the cell-cavity
into the thickened cell-walls, or as hollow prolongations of the
cells into the intercellular substance. In the first case, they
might be compared to the porous canals of vegetable cells ; in the second, they would correspond with prolongations of
cells, such as we shall often again meet with in the progress of
this work. Meanwhile, for an example of those cells which
are extended out on all sides into canals, and which I have
called stellated cells, the reader is referred to plate II, figs. 8
and 9, where those transformations are delineated from pigment-cells. I decidedly give the preference to the latter explanation of the canaliculi, because they pass through the entire thickness of the firm cartilaginous substance, a fact which, in order
to be consistent with the first view, requires for its explanation that the substance between the cell-cavities should be formed of the thickened cell-walls, which is certainly not the case
in the cartilages of mammalia, as is seen in plate III, fig. 2.
The osseous corpuscles, with their canaliculi, would therefore
be the cartilage-cells transformed into stellated cells, and filled
with earthy matter. We shall return to this metamorphosis of round into stellated cells when treating of the pigment. The
resemblance between stellated pigment-cells and osseous corpuscles is sometimes very striking, as is shown, for example,
by the pigment-cell which lies to the extreme right in plate II,
fig. 9. The compact bony substance is intercellular substance ;
it is, however, probable that the walls of the stellated osseous
cells form some, if only a very small part, of it.
When ossification takes place, the earthy matter is first deposited in this intercellular substance, and probably at a sub- sequent period also in the cell-cavities. The deposition often causes the substance to assume a darkish granulous appearance in the first instance, which it afterwards loses, becoming more equally dark. If we assume, what is extremely probable, that the earthy matter is contained in bones in combination with the cartilaginous substance, in a manner analogous to a chemical union, and not in the form of minutely-divided granules, the mode in which the union with the earthy salts takes place may then be explained in two ways: either the earthy matter combines with a particle of cartilaginous substance in such a manner that each smallest atom receives in the first instance a
