INTO CONTINUOUS TISSUES. 93 
cells of the tissues of the following classes, in most instances, 
do not advance beyond this early stage in the development of 
the feather-cells, but the changes necessary to the formation 
of the subsequent tissues occur at this period; their cellular 
nature is, however, quite as certain as is that of the young 
feather-cells, although it be not possible to recognize their cell- 
wall so clearly as in their perfectly-developed condition. 
The matter contained in the cells is either a transparent 
fluid, as in the cells of the pith of feathers previously to their 
becoming dry, or in the crystalline lens, when it contains 
albumen ; or, a minutely-granulous mass, as in many epithelium- 
cells, or pigment-granules ; or, it is altogether absent, and the 
cell-walls, im consequence of their flattening, are in immediate 
contact. The air in the cells of the pith of mature feathers 
simply penetrates from without, durmg the process of their 
desiccation. With the exception of some of the cells of the lens, 
- all the cells of this class are invariably furnished with a nucleus 
of the characteristic form. It is not, however, a persistent 
structure, as in the previous class, but in very many instances 
becomes absorbed when the cells have reached maturity ; such 
is the case in the pith of the feather, the superior laminz of 
the epidermis, the nails, crystalline lens, &c. &c. 
As a general rule the cells remain independent during all 
these changes, that is to say, each cell retains its especial wall, 
and its own peculiar closed cavity. More or less complete 
blendings of the cell-walls, and even of their cavities also, 
occur, however, as exceptions even in this class. The epithelial 
scales of the nail are so intimately connected together, that it 
is rarely possible to trace the contour of one of them in its 
entire circumference; and the same appears to be the case 
with the epithelium in the vessels of the adult. The coalescence, 
however, does not appear to be perfect, for, by the employment 
of concentrated acids, the scales of the nail may be separated 
somewhat more readily from each other. A union of the 
cavities of several cells seems to occur in the pigment-cells. 
A prolongation of a cell filled with pigment may be seen to 
pass uninterruptedly to the cavity of another cell (plate II, 
fig. 9, a). In such an instance, probably, the prolongations of 
two cell-cavities join at a certain point, the cell-walls unite 
together there, and the partition-wall becomes absorbed, and 
