20 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 neighbour- 
ing cells we see much more intercellular substance. 
The cells at first appear finely granulated, and not so trans- 
parent as in the more fully developed condition. The thick- 
ening 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 granu- 
lous and indistinctly circumscribed mass, the rest of the inter- 
cellular 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 sur- 
rounding that structure has obtained a defined external boun- 
dary, 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 lymg 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 homogeneouswith 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 con- 
densed 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 | 
