COMPARATIVE RETROSPECT. 107 
It is easy to see which elements of the tissues of this and 
the preceding class correspond. There the whole tissue 
consisted of cells, closely crowded together, and the in- 
tercellular substance was almost nil. Here we find the like 
arrangement only in the lowest stage of development of the 
most simple cartilages. In such as are more highly developed, 
those of all the mammalia for example, the cells lie surrounded 
by a larger quantity of intercellular substance, which forms 
the proper cartilaginous substance ; but the cell-walls contri- 
bute only very slightly, or not at all, to its formation. The 
proper firm substance of these higher cartilages, therefore, has 
its analogy in the former class, only in the minimum of cyto- 
blastema by which the cells are connected, while, on the other 
hand, it corresponds with that which, in the first class, was 
the fluid, wherein the isolated cells were formed. The carti- 
lage-cells in this class, however, correspond precisely to the 
epithelium-cells, the feather-cells, &. &c., in the preceding 
one, and the blood-corpuscles, mucus-corpuscles, &c. in the 
first class. 
We have not found any new changes in the form of the 
cells in this class. Most of them were angular, somewhat 
approaching the circular form; and stellated cells, so far at 
least as we may be permitted to regard the osseous corpuscles 
as such, were also frequently met with. (See pp. 29, 30.) 
Some cells, which were remarkably elongated, were observed 
near to the surface of several cartilages, in which situation 
they are known as greatly elongated cartilage-corpuscles ; 
still, however, this appearance is never presented by the cells 
of this class in so remarkable a degree as it is by those 
of the crystallme lens in the previous one. The fibro- 
cartilages, on the other hand, form the immediate transition 
from this to the following class, for in them a bundle of fibres 
seems to be formed out of each cartilage-corpuscle, a process 
which we shall consider more minutely when treating of cel- 
lular (areolar) tissue in the next class. 
We have observed the formation of cells around the pre- 
viously-existing nucleus, and their progressive growth, going 
on in this class in a similar manner to that exhibited in the 
preceding, and the true cartilage-cells were also seen to form 
around a cytoblast which. lay external to the cells already 
