16 STRUCTURE AND GROWTH 
that the intermediate substance of the cell-cavities is not a 
simple structure, but one composed of the walls peculiar to the 
contiguous cells: that is to say, each cell is surrounded with a 
thick ring, its peculiar wall, the external outline of which is 
more or less distinct. In the preparation from which the deli- 
neation is taken, it was in some parts quite as distinct as the 
internal. Between two cells these external outlines blend into 
one line, but separate again when the contact of the cell-walls 
ceases ; there is thus often left between the cell-walls a three 
or four-cornered intermediate space (c), filled with a kind of 
intercellular substance. No other structure, no deposition of 
strata, or distinction between primary cell-membrane and se- 
condary deposit can be observed in the thickened cell-walls. 
The cell-contents also remain clear after the thickening of the 
walls. At the base of the branchial ray, it is scarcely possible 
to distinguish between the different cells-walls, and the cartilage 
presents the appearance of a homogeneous substance, in which 
separate small cavities only are seen. (Pl. I, fig. 7.) Around 
some few only of the cell-cavities, a trace of the peculiar cell-— 
walls may be seen in the form of a ring. ‘This ring is usually 
somewhat thin, so that the entire intermediate substance of the 
cell-cayities cannot be formed of the cell-walls ; but the inter- 
cellular substance, which was so small in quantity in the centre 
of the branchial ray, here contributes essentially to the forma- 
tion of the cartilaginous substance, and often completely pre- 
vents the immediate contact of the cell-walls. This intercel- 
lular substance appears, however, to be homogeneous with that 
of the cell-walls, and in most situations coalesces with them. 
The cell-cavities, which are here transparent and without gra- 
nulous contents, are now the cartilage-corpuscles. 
The process of formation of this cartilage is as follows. It 
consists originally of cells, which lie in very close contact, but 
every one of which has its special, very thin cell-membrane. 
This follows, firstly, from the complete accordance in appear- 
ance, of cartilage in its earliest stage, with vegetable cellular 
tissue; secondly, from the presence of the nucleus in the 
young cells of cartilage, a structure which, as will subsequently 
be seen, occurs in almost all the cells proved to exist in other 
tissues ; thirdly, from the fact, that a separation of the cell- 
walls is often distinctly perceptible in instances where they are 
