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 bv the 

 matrix and hardened in the air, the view is manifestly an 

 erroneous one ; what Ave 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. Bnt 

 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 

 hornv tissue, the nutritive fluid must be furnished bv the 

 nearest substance in which blood-vessels exist, and in this 

 sense the nearest organized substance may be called, matrix. 

 But whether this cvtoblastema 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 onlv in the cvtoblastema 

 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 Ions: 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 



