is STRUCTURE AND GROWTH 



Tlie walls of the cells appear to contribute little or nothing to 

 the formation of the substance of most of the ossifying carti- 

 lages, — those of the higher animals for example. 



The cartilages of the branchial arches of the tadpole, like 

 those of the branchial rays of fishes, consist of cells, which are, 

 however, much larger than those of the fish, though smaller 

 than the cells of the chorda dorsalis, with which they have, in 

 every other respect, much similarity. The partition-walls of 

 the cells are thicker than in the chorda dorsalis, but they may 

 still be termed thin when compared with the cell-cavities. (See 

 pi. Til, fig. 1, which exhibits branchial cartilage from the young 

 larva of Pelobates fuscus.) The cartilage intended to be used for 

 investigation must be taken quite fresh from the living animal ; 

 for the structures become very indistinct if it be allowed to lie 

 in water for any time after death, even though it be entire. 

 After stripping off the mucous membrane, the cellular structure 

 is readily recognized by the aid of the microscope. The cells 

 vary much in size, and are more or less flattened against one 

 another. The wall of each separate cell may be distinctly seen 

 in the majority of instances, and its thickness might even be 

 measured ; that we cannot trace it so evidently in the smallest 

 cells is probably referrible to the extreme thinness of their 

 sides. The walls of the cells are for the most part in contact, 

 but intercellular substance may be seen in many situations, 

 and especially where several cells are contiguous. The surface 

 of the cartilage, which is represented on the left and lower 

 margin of the figure, (pi. Ill, fig. 1,) is formed in the first place 

 of intercellular substance, which, in as much as the cells ori- 

 ginate in it, may be called Cytoblastema. 



This cartilage may, therefore, be described as consisting of 

 intercellular substance, or cytoblastema, in which great num- 

 bers of cells are seen. The cell-contents are generally clear as 

 water ; but in the younger and smaller ones (for example, c } ) the 

 contained matter is less pellucid, and somewhat granulous. Each 

 cell contains a spherical granulous nucleus, which lies upon the 

 inner surface of the wall, and which again encloses a nucleolus. 

 The size of the nucleus is not precisely alike in all the cells : it is 

 somewhat larger in the large ones, but its size bears no proportion 

 to the increased bulk of the cell ; and again, the smaller cells 

 are not much larger than the nucleus which they contain. 



