STRUCTURE AND GROWTH
this fact explains the common appearance of two or four cells lying together in a group, being separated from one another by thin walls, whilst between such groups and the neighbouring cells we see much more intercellular substance.
The cells at first appear finely granulated, and not so transparent as in the more fully developed condition. The thickening of the cell-membrane takes place simultaneously with its expansion. One of the cells in pl. III, fig. 1, exhibits two nuclei, one of which, like those of all the other cells, has but one nucleolus, the other having two. It may be conjectured, that this second nucleus is destined to the formation of a young cell within the larger one.
In the intercellular substance at e in the same figure (pl. III, fig. 1,) may be seen a small corpuscle, surrounded by a granulous and indistinctly circumscribed mass, the rest of the intercellular substance being smooth and homogeneous. This is, perhaps, a nucleus in the act of formation, the nucleolus of which is already developed ; and when the granulous mass surrounding that structure has obtained a defined external boundary, it will form a nucleus. If such be the case, we have here an instance of accordance of the development of the germ itself with the formation of the nucleus of vegetable-cells ob- served by Schleiden.
On examining the cartilage of the branchial arches of the tadpole in the more completely developed state, (pl. I, fig. 8,) we find the cells generally lying in groups, so that two, three, or four lie close together, separated from other groups by thicker partition walls. The special walls of the individual cells are less distinct, but at several spots where three or more cells are in contact, for example, at a, the separation of the walls may yet be seen, and a trace of intercellular substance is also present ; the latter, however, is almost homogeneous with the cell-walls. It may also be observed that the cell-walls are thicker in these situations than they are represented in pl. III, fig. 1. Some parallel lines may be seen at various spots in these condensed cell-walls, and the thickening might, in such instances, be supposed to be really produced by a stratified deposition of the substance upon the internal surface of the cell-wall. But at the same time it must be remembered, that every partition-wall between two cells must consist of two layers, each of which
