30 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 LI, 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 thick- 
ness 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 cor- 
puscles is sometimes very striking, as is shown, for example, 
by the pigment-cell which hes 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 de- 
posited 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 stance, 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 che- 
mical 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 
