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 it 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 



