OF CARTILAGE. 23 
cells is subsequently absorbed. After the cartilages of the 
branchial rays of fishes have been exposed to the action of water, 
it is only in the young cells that the nuclei are visible; they 
are much more rarely seen in those cells of which the walls are 
already very much thickened. In many cells of the branchial 
cartilages of the tadpole, a small nucleus with aragged outline 
may be observed, which is probably the cytoblast of the cell 
in the act of undergoing absorption. These cytoblasts (nuclei) 
of the true cartilage-cells always lie in the cell-cavity, even 
when its wall is thickened, and it is impossible to distinguish 
whether they lie free or are still connected with the cell-wall. 
A twofold explanation is here possible: either the cytoblast 
separates from the wall after the formation of the cell-membrane 
is perfected, and falls free into the cavity (as occurs in plants), 
and at such period a secondary deposition of substance upon 
the cell-wall first commences ; or the thickening of the wall is 
due to an actual increase of the original cell-membrane, and in 
that manner the nucleus is pushed inwards, and may remain 
in connexion with the wall. If a secondary deposition of 
substance took place before the nucleus was disengaged from 
the cell-membrane, that body must be enclosed in the wall, 
and would not lie in the cell-cavity. As both these expla- 
nations are possible, it will be seen that no conclusion can be 
drawn from the position of the nucleus, as to whether the 
thickening of the cell-wall be a secondary deposition, or an 
actual growth of the cell-membrane. Sometimes a carti- 
lage-cell presents more than one nucleus; when in such a 
ease the original nucleus of the cell is absorbed, all those 
observed are probably the germs of new cells, which have not 
as yet commenced their development. The same fact is fre- 
quently observed in plants. The nuclei in the branchial 
cartilages of the tadpole have for the most part the same size ; 
some, however, which are probably not as yet perfectly formed, 
are smaller than others. It also often occurs that a nucleus 
is seen expanded to three or four times the usual size ; such 
instances might be mistaken for young cells without nuclei, 
but they may be readily recognized by their general aspect. 
They are more transparent and delicate, and exhibit one or 
two nucleoli, which are easily detected ; when two are present 
they are widely separated from one another. According to 
