COMPARATIVE RETROSPECT. 100 



blending be so complete that it cannot in any way be dissolved, 

 the simple fact being, that the cell-walls arc 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 exclusivclv, 

 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 process 

 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 elementarv cells of 



m 



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 alwavs been 

 drawn between the growth of the organized and that of the 

 unorganized structures ; and much had alreadv been said 

 in 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 



