INTO CONTINUOUS TISSUES. 95 
dependent upon a secretion of the so-called matrix. If by 
this it is meant that the substance of horn is secreted by the 
matrix and hardened in the air, the view is manifestly an 
erroneous one; what we call horny substance being either 
merely the cell-walls, when, for example, the cells are flat, and 
there are no cell-contents, or the cell-walls and cell-contents 
together, when the cells are polyhedral, as in hoofs. All these 
cells are independent structures, which grow organically. But 
if, by the above description, it is meant that the organized 
matrix only furnishes (or secretes) the cytoblastema, no im- 
portant objection can be raised. The cells of the horny tissue 
require a nutritive fluid for their growth. This is supplied to 
them by the blood, as it is in all tissues. As, however, the 
blood-vessels themselves do not pass between the cells of the 
horny tissue, the nutritive fluid must be furnished by the 
nearest substance in which blood-vessels exist, and in this 
sense the nearest organized substance may be called, matrix. 
But whether this cytoblastema which exudes from the matrix 
have a specific character, and on that account horn-cells are 
formed in it—or whether their formation take place in it for 
the same reason that the muscle-cells, those of areolar tissue, 
and so on, originate in other parts of the body, that is to say, 
whether it is determined by the plan of the entire organism, 
—is a question which does not as yet admit of a decision. It 
is, however, a characteristic of all the cells of this class (with 
the exception of the crystalline lens, which I have not examined 
in reference to the point), that the new cells are not generated 
between those already formed, but only in the cytoblastema 
nearest to the organized substance, if not, indeed, always in 
immediate contact with it. The teeth were necessarily sepa- 
rated from this class, because, as we shall see hereafter they 
present quite a different relation of the cells. The new cells 
of cartilage, so long as it does not contain any vessels, are not 
only formed upon the surface of the tissue, but also between 
the most recently-formed cells. 
The chorda dorsalis forms the transition from this class to 
the following one. The cell-walls remain separate in the 
highest stage of their development, and it is only in their 
rudimentary forms, in the osseous fishes for example, that they 
