CRYSTALLINE LENS. 89 
They have much the same course as the others, that is, they 
form arches from the anterior towards the posterior surface. 
They do not, however, reach the axis either in front or behind, 
but the fibrous zone is thickest in the middle, gradually dimi- 
nishes towards the anterior and posterior surfaces of the lens, 
and terminates altogether without the fibres meeting anywhere 
in front, or reaching the axis. No laminated structure can 
be perceived in the zone; but the fibres may be readily imsu- 
lated throughout its entire breadth. When the ends of these 
fibres are examined, they are found to be either simply rounded 
off, or to terminate in a small round dilatation, or to pass over 
into larger globules (cells) ; or, on the contrary, it may be more 
correctly expressed by saying, that the larger globules or cells 
become elongated to these fibres (see pl. I, fig. 12). The 
transition from cells to fibres may either be very gradual or 
somewhat sudden ; but even in the latter case, the contour of 
the cell passes immediately over into that of the fibre, so that 
the latter is not merely affixed to the globule, but is a true 
continuation of it. Now, these cells which become elongated 
into fibres, perfectly accord with other neighbouring cells which 
are as yet quite round; and these again accord with the cells 
forming the greater portion of the lens in the embryo chick. 
They are round, extremely pale, smooth, transparent cells of 
very various size (see pl. I, fig. 10). Some have a very beau- 
tiful, sharply-defined, oval nucleus, which, in most instances, is 
not flattened, and which lies upon their wall, and encloses one 
or two nucleoli. Some cells are scarcely larger than the nu- 
cleus which they contain, fig. 10, 5, for example. Some of 
these enclose young cells (fig. 10, d), and as they may be ob- 
served to flatten against the wall of the parent-cell, there would 
seem to be no question about the existence of a special cell- 
membrane for the latter, and thus the true cellular nature of 
these globules appears indubitable. The presence of the nu- 
cleus, and the fact of the outlines of the cells being too sharply 
defined for mere shadows, would, however, have been sufficient 
to render their cellular character probable. The very distinct 
nucleoli contained in the nuclei, which are not flattened, lie 
upon the inner surface of the wall and not in the centre, as 
represented in fig. 11. 
Since, then, the round cells, as we have seen in the chick, 
