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. Pl. I,fig. 8, ff, represents such young cells from the branchial cartilages 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 commonly 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 only simple 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 increase 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, b.
The nucleus of the true cartilage-cells like that of vegetable-
