18 STRUCTURE AND GROWTII 
The 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 
pl. ILI, 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 le 
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, (pl. III, 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. Hach 
cell contains a spherical granulous nucleus, which lies upon the 
mner surface of the wall, and which again encloses a nucleolus. 
The size of the nucleus is not precisely alike in al! 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. 
