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 diminishes 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 beautiful, 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, b, for example. Some of these enclose young cells (fig. 10, d), and as they may be observed 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 nucleus, 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,
