COMPARATIVE RETROSPECT. 109 
blending be so complete that it cannot in any way be dissolved, 
the simple fact being, that the cell-walls are no longer dis- 
cernible with the microscope. I shall not bring forward the 
splitting of -the dental fibres as examples here, nor indeed 
make any reference to the teeth in this retrospect, their ex- 
planation being as yet too problematical. It has, however, 
been already mentioned as a doubtful point, whether a coales- 
cence of the walls actually takes place in all cartilage-cells, 
for instance, in those of the higher animals. 
Ossification appears to occur especially, perhaps exclusively, 
in those cartilages which have a greater quantity of intercel- 
lular substance. It consists probably in a chemical union 
between the calcareous salts and the firm portion of the car- 
tilage substance. In the first commencement of the process 
the cartilage frequently acquires a granulous appearance, which 
subsequently disappears, the entire substance meanwhile be- 
coming gradually dark. At the same time the cartilage-cells 
undergo a transformation into the osseous corpuscles, a proéess 
which must probably be explained as analogous to the for- 
mation of the stellated pigment-cells. There is reason to 
suppose that the osseous corpuscles and the canaliculi which 
issue from them, also become filled with earthy salts by the 
calcifying process. 
The class of cells now under consideration has yet another 
point of especial interest for us, since it is the first in which 
organized structures, that is, structures provided with vessels, 
occur. The accordance between the elementary cells of 
‘unorganized animal tissues and vegetable cells might be con- 
ceded, without granting a connexion between the organized 
tissues (which are especially characteristic of animals) and 
the structure of vegetables. A distinction had always been 
drawn between the growth of the organized and that of the 
unorganized structures; and much had already been said 
im a general way about a vegetative growth of the non-vascular 
structures, the crystalline lens for instance, though the analogy 
which existed between their elementary particles was not 
proved. Cartilage, then, is the first structure which teaches us 
that a tissue, which, at a later period at least, contains vessels, 
is composed of cells, perfectly according, in their development, 
with those of plants; and, therefore, that a similar formative 
