22 STRUCTURE AND GROWTH 
which none of these secondary cells, completely filling the 
parent one have been developed, contain very commonly 
several such nuclei, and also one or more young cells. PI. I, 
fig. 8, ff, represents such young cells from the branchial carti- 
lages of the larva of Rana esculenta. They are round vesicles 
containing a nucleus identical in form and size with those 
which lie free, but which is situated upon the internal surface 
of the wall, and never in the centre of the cell. This nucleus 
is never wanting in the young cells. The cells, however, vary 
much in size, some being scarcely larger than the nucleus they 
contain, others twice or thrice as large: From one to three 
such young cells, in various stages-of development, are com- 
monly found within the primary one, where they sometimes 
become flattened from want of space. As the figure represents, 
most of the secondary cells contain these young ones, and but 
few of them onlysimple nuclei (such as have no cell around them), 
in some of the young cells, indeed, a second somewhat paler 
nucleus appears. These young cells lie free within the primary 
cell, and may be isolated in the same manner as was described 
with regard to those of the chorda dorsalis. They appear in the 
first instance to be perfectly transparent ; but gradually obtain 
a granulous yellowish aspect, and it is remarkable, that the 
earliest formation of this yellowish deposit takes place generally 
if not constantly, in the neighbourhood of the nucleus. 
It will thus be seen that these young cells, (fat cells ?) which 
are formed within the true cartilage-cells, furnish us with a 
series of observations as regards their development, similar to 
that observed in the formation of the cartilage-cells themselves: 
namely, simple nuclei, cells closely encompassing those nuclei, 
and all the stages of transition up to the largest cells; but 
never have we met with these young cells without nuclei. So 
that the same conclusions might be arrived at with respect to 
the mode of their development, as were before with regard to 
that of the cartilage-cells, namely, that the nuclei are first 
formed, and around them the cells, precisely as in plants. The 
nucleus in these young cells, however, does not appear to in- 
crease in growth after the cell has once formed around it. 
The accordance in form between these and the young cells of 
vegetables is shown by comparing Plate I, fig. 8, with fig. 2, 6. 
The nucleus of the true cartilage-cells like that of vegetable- 
