Cuap. II. - HISTOLOGY. 607 
beneath it. Some of the cartilage cells do not fill the cavity in the matrix; and 
in such cases (fig. la, fig. Ib) the thick cell wall (4) may be seen passing across 
the cavity, sometimes at one end and sometimes at both. Close upon this, there 
rests a dense, fibrous layer, (fig. 1, 6, ¢, d, e, f, fig. 2, b, ¢, d, e, f,) containing cells 
in various stages of development. The innermost of these cells (fig. 1, ¢) are 
irregularly round and very flat, (fig. 2, ¢,) so as to appear almost like dark lines 
in a transverse section, and each contains a rather faint, granular mesoblast; other- 
wise the contents are homogeneous. The irregular, wavy processes from these cells 
connect here and there with those of neighboring cells, and appear to be spaces 
in the fibrous substance; but whether they are direct prolongations from the cell 
could not be made out with certainty, although it appeared very probable that such 
is the case. A little further toward the surface of the bone, these cells are found to 
be elongate-oval (d) or oval, and not quite so flat (fig. 2, d) as those last men- 
tioned, and the wavy processes are shorter, but the mesoblast is the same. Still 
further outward, the cells (fig. 1, e) are very much elongated, irregular in outline, 
and are as thick (fig. 2, ¢) as they are broad. Their mesoblasts have altogether 
disappeared, and the wavy prolongations are very faint. At the surface of the 
middle line of the rib, the cells (fig. 1, /) are very long and slender. At the edge 
of the rib (fig. 1, y) the surface cells are like those at the middle line, but they 
are much more slender. They cannot be distinguished very readily from those in 
the soft, fibrous corium, except that the latter are in a yielding matrix, whilst 
the former are surrounded by a hardened substance. The fibres (g') of this 
outermost layer, as well as those (/') of the corium, are composed of rows of 
minute granules. The fibres of the more interior layers are very faint. As 
these investigations were made upon perfectly fresh bones, and no reagents were 
used, it is clear that the mesoblasts of the cells in the fibrous layers are per- 
fectly normal, and fully show that bone lacune, with their characteristic canaliculi, 
are nothing less than these mesoblasted cells, with their wavy processes anasto- 
mozing -with each other. 
The Limbs. Soon after the shield begins to form, and the feet begin to show 
external signs of the toes, (Pl. 25, fig. 11, and p. 565,) the bones of the feet, 
or rather their cartilagmous matrices, are quite conspicuous. The terminal or 
claw bones have not yet appeared. The matrix of each bone is composed of 
very irregular cartilage cells (Pl. 21, fig. 22, 24). Those in the centre are set 
at a considerable distance from each other, but as they approach the surface of 
the matrix they approximate, and at the same time gradually assume the smaller 
size and form of those in the surrounding tissue (fig. 25). Each cell contains 
fine granular contents and a large, round, clear mesoblast, which contains one 
or two entoblasts (fig. 22a). A short time before the Turtle is hatched, the 
