92 INDEPENDENT CELLS UNITED 
This prolongation of cells into long cylinders (called fibres) 
is much more remarkable in the crystalline lens. The fibres 
or cylindrical cells of the lens, however, themselves undergo 
very important modifications, inasmuch as they often become 
flattened on two sides into bands, and then the margins of 
these bands become denticulated. This serration is probably 
produced by a more forcible expansion, and therefore bulging- 
out of the walls of these bands at different points, which follow 
each other at pretty regular distances, whilst the intervening 
points, situated close to them, remain stationary. All the dif- 
ferent stages of this serration, may be observed in the lens of 
the fish, if the fibres are examined from the exterior towards 
the centre of the structure. Now, in this flat and serrated 
condition, the cells of the crystalline lens perfectly resemble 
those of the epidermis of some grasses, and this accordance 
with indubitable vegetable cells is a proof that, despite the 
modifications which they undergo, they do not lose their 
cellular character. If the explanation I have given of the 
mode in which the serration is produced be correct, it will not 
materially differ in principle from the elongation of the cells 
into cylinders and fibres. For, in the latter case, a more 
forcible expansion of the cells is likewise presumed to take 
place in certain situations : the sole difference being, that in 
the latter case it takes place only at one or two opposite points 
of a cell, whereas with the serration if occurs at many sepa- 
rate ones. At this stage of our inquiry, we are reminded of 
the form of many pigment-cells, in which this expansion of the 
cell, at certain spots, takes place on several sides, and in a far 
higher degree, causing the cell to assume an irregular stellated 
form. The prolongations of these cells, however, retain their 
character as hollow processes, even when almost as minute as 
the fibres of cellular (areolar) tissue. 
The distinction between cell-membrane and cell-contents is 
nowhere more distinctly defined than in the fully-developed 
cells of this class. In the perfected cells of the pith of 
feathers, for example, it is as marked as we ever find it to 
be in plants. When traced backwards to their earliest stages 
of development, their true cellular formation scarcely admits 
of a doubt, although the cell-membrane, for reasons given at 
page 36, cannot be so clearly distinguished. The elementary 
