Structure and Growth in Animals and Plants, 23 



pends, in part, upon the thickening of the walls of the cells, and partly 

 on the intercellular substance. In the cartilages of the higher animals, 

 the thickening of the walls of the cells has not been observed. The prin- 

 cipal mass of the future cartilage appears to belong to the intercellular 

 matter, which comprehends many generations of cartilaginous cells. 



We may observe, in the branchial cartilages of the tadpole oi Pelobates 

 fascus, a mode of developing the cells analogous to that of plants. Some 

 of these cells contain simple nuclei, others contain smaller cells provided, 

 in like manner, with a nucleus at their internal wall, and but little exceed- 

 ing in thickness that of this nucleus ; others, again, containing still larger 

 cells than the latter ; so that we may here find all the degrees of transition. 



The mode of the formation of cartilage takes place, it would ap- 

 pear, without the participation of the vessels, in a manner analogous to 

 the growth of plants. 



With regard to the radiated corpuscles (corpuscula radiataj of the 

 bones, which become apparent after ossification, the mode in which their 

 canals are produced is not yet very clearly ascertained. According as 

 we regard the cartilaginous corpuscles as cavities of the cells, whose 

 walls, by becoming thickened and confounded with the cellular substance, 

 would constitute the cartilage ; or, according as we conisder these cor- 

 puscles as entire cells ; while the intermediate substance of the cavities 

 of the cells would be nothing else than the intercellular substance ; these 

 rays would be, according to Schwann, cither small canals penetrating the 

 cellular cavities in the thickened walls of the cells, or prolongations of the 

 cells in the intercellular substance. In the first case, these minute canals 

 might be compared with the porous canals in the cells of plants ; in the 

 second case, they would answer to the prolongations of these latter. 31. 

 Schwann regards this last opinion as most likely to be the correct one. 



Besides the formation of young cells in the interior of those already 

 existing, Schwann further distinguishes, in animals, the production of new 

 cells outside of these, in a substance without structure, namely, the cytO' 

 blastema. It is usually the nucleus that appears to be developed first, 

 and then the cell around it. 



f9 In many animal tissues new cells appear on the outside of the cells already 

 formed. In one instance the cytoblastema is interior, in another exterior. 



Schwann's observations on the ovula, considered as a cell, have yielded 

 the following results : — 



The ovula, contained in the follicle of Graaf, is enclosed in a layer of 

 granules, which are cells having a nucleus on their inner face, with one 

 or two smaller nuclei (corpuscules du noyau). The cells originate in the 

 liquid of the follicle of Graaf, as in a germinative matter. It is easy to 

 comprehend how these cells, possessed of independent life, become deve- 

 loped when they arrive, along with the ovula, in the uterus, in order to 

 form other tissues — the chorion, for example. 



The ovula everywhere possesses an external membrane without struc- 

 ture, whether it be the chorion or the vitelline membrane ; the ovula, 

 therefore, is always a cell. The viteUiue cell incloses the vitellus, and 



