108 COMPARATIVE RETROSPECT. 



developed. On the other hand, a formation of cells also takes 

 place within the true cartilage-cells, but it is probable that 

 they have a different signification from those within which they 

 are generated. A deviation from the previous class seemed to 

 occur with respect to the spot at which the young cells are 

 formed, in relation to the entire tissue. In the former class, 

 so far as we could perceive, they were formed at that part 

 only where the tissue was in immediate contact with the 

 organized substance. The formation of the new cells in 

 cartilage, it is true, did not take place throughout the entire 

 thickness of the tissue, but (so long at least as the cartilage 

 itself is not furnished with vessels) only near the sur- 

 face, and therefore, at the spot where it was in contact with 

 the organized substance ; still, however, it not only took 

 place at that point of contact, but went on also between 

 the cells most recently formed, as if cartilage had a greater 

 capacity of imbibition, so that the cytoblastema penetrating 

 from the blood-vessels into the parenchyma arrived at the 

 deeper seated portions of the tissue more speedily; and, there- 

 fore, retained its fresh plastic force even in that situation; 

 or, as if the cartilage itself possessed a higher vitality, and, 

 therefore, the cytoblastema retained its productive power for 

 a longer period, although penetrating quite as slowly as in 

 the previous class. 



Although the modifications in the form of the cells of this 

 class vary but slightly from those of the preceding one, yet we 

 see two striking changes in the cells and their cytoblastema, 

 namely, the coalescence of the cell-walls and ossification. The 

 thickening and transformation of the cell-walls were very dis- 

 tinct in the last class, for example, in feathers. Here a still 

 more strongly- marked thickening of the cell-walls takes place 

 in several cartilage- cells. The external contours of the walls, 

 however, gradually disappear in such instances, and a coales- 

 cence takes place to such an extent as to leave merely the 

 cell-cavities perceptible, lying in an homogeneous substance. 

 The blending of the cell-walls takes place either between the 

 walls of neighbouring cells, in instances where they are in 

 immediate contact, or, with the intercellular substance, when 

 the cells are surrounded by it. Further investigations are re- 

 quired in order to decide the question as to whether this 



