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 j 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 j or, a minutely- granulous mass, as in many epithelium- 

 cells, or pigment-granules ; or, it is altogether absent, and the 

 cell-walls, in consequence of their flattening, are in immediate 

 contact. The air in the cells of the pith of mature feathers 

 simply penetrates from without, during 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 ; sueh 

 is the case in the pith of the feather, the superior laminae 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 

 Mendings 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 



