in their independent state, is very difficult, from the circumstance of our being unable at that period to distinguish between them and the surrounding tissues; for a whole organ is then composed entirely of independent cells, which have not as yet undergone any transformation. It is true I saw an independent cell, furnished with a nucleus, which seemed to have separated from the nervous cord, in one of the preparations alluded to, fig. 6 b; but I cannot positively assert that it had actually separated from that particular part, nor that it was a primary nerve-cell, for the cells in that preparation had not as yet undergone any change. In this instance, therefore, we must content ourselves, for the present at least, with the analogy to muscle.
These fibres, or secondary nerve-cells, differ very much in their appearance from the subsequent nervous fibres, which are furnished with distinct but not dark outlines; they have a pale, granulated aspect. By progressive development, however, they become converted into the white fibres, and pl. IV, fig. 8, d, represents the transition. The part of the figure to the right hand exhibits the fibre yet in the early condition, pale, granulated, and furnished with a cell-nucleus; in the portion to the left, it has completely assumed its subsequent form: it has a dark outline, is not granulated, and the one portion passes immediately into the other. The identity be- tween these pale fibres and the subsequent white nervous fibres is thus established.
In what then does this transformation of the pale granulated fibres into the white fibres consist? Clearly in the development of the white substance; we may, however, imagine three different modes in which this development may take place. It may take place, 1stly. By the white substance form ing as a sheath (cortex), around each fibre, and in this manner enclosing it. By this mode of explanation the fibre would be identical with the pale band discovered by Remak, which would therefore be the cell-membrane itself. 2dly. The white substance might be regarded as a transformation and thickening of the cell-membrane of those fibres, or secondary nerve-cells. According to this view, the white substance would be the cell-membrane, and Remak’s band the firm contents of the secondary cell. 3dly. The white substance may be formed as
