Intercellular Substance of Cartilage, 
99 
shrinks and undergoes condensation. Near the surface^ in 
many cartilages, a layer exists which could not be distin- 
guished from embryonic cartilage, and here the changes 
occurring during development may be readily studied. 
Of the Formation of the 'Cells within Cells' 
As growth continues in the higher forms of cartilage, the 
masses of germinal matter already separated from each other 
by a considerable thickness still continue to divide and sub- 
divide, but more and more slowly, and at the same time con- 
densation still proceeds in the matrix already formed. There 
are, therefore, collections of masses of germinal matter separated 
from each other by thick layers of formed material, and there 
are the individual masses constituting each collection separated 
from each other by a thin layer of formed material. Each of 
these may divide and be separated by a still thinner layer of 
formed material. Thus are formed the ^cells within cells.^ 
The septa do not grow in, but the living germinal matter 
simply divides, and its outer part undergoes conversion into 
the formed material or matrix. 
Of the Structure of a very simple form of Cartilage, 
But in many cartilages of the frog the structure is not 
complicated by the formation of these secondary and tertiary 
formations (cells within cells). The cartilage in its fully 
formed state consists merely of germinal matter, which passes 
into imperfectly formed matrix, and this is continuous with 
the fully formed cartilaginous tissue. (Figs. 2, 3, 4.) Here 
the continuity of structure can be most distinctly demon- 
strated. If, however, the tissue be kept for a short time after 
death, and more especially if it be placed in water, a clear 
interval between the matrix and the so-called cell soon makes 
its appearance, and we observe the characters, so often repre- 
sented in figures, which have led most observers to infer the 
existence of a cell- wall distinct and separate from the matrix. 
If cartilage be kept for a longer period, the germinal matter, 
and soft imperfectly developed formed material, break down 
and become liquefied, and then the cartilage appears as a 
matrix containing numerous vacuoles or spaces, an appearance 
which has led to the view that cartilage and many other tis- 
sues result from the ^ vacuolation' of a plasma or matrix. But 
there are no 'vacuoles^ during life. These spaces were occupied 
by the living active matter, which was alone concerned in the 
formation and growth of the cartilage-tissue (matrix) . 
