Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/114

CRYSTALLINE LENS. mode of life has been considered as generally resembling that of vegetables. We shall find the latter to be the correct view, and the singularity of the mode of its nutrition disappears altogether, when we become acquainted with the fact, that the growth of the organized tissues resembles that of vegetables. The general statement, that the lens has the vitality of a vegetable, does not, however, express much, unless the relation of its elementary structure to the cells of plants be proved. The lens is known to be composed of concentric layers, made up of characteristic fibres, which, not to go into details, may be said to pursue a general course from the anterior to the posterior surface.

In order to become acquainted with the relation which these fibres bear to the elementary cells of organic tissues, we must trace their development in the foetus. When the lens of a chick is examined after eight days’ incubation of the egg, no fibres are to be found; but it is composed of round, extremely pale, and transparent smooth cells. Some contain the characteristic cell-nucleus, in others it cannot be detected; and there are also many nuclei without surrounding cells. Some larger cells may be observed in the chick at a more advanced period, which contain in their interior one or two smaller ones (see pl. I, fig. 10, d, from a foetal pig), and from the manner in which these cells become flattened against the wall of the parent-cell, as well as from the presence of the nucleus in other cells, we may conclude, that these pale globules are actually cells, al- though a cell-membrane be not distinctly recognizable. Werneck, who first observed them, likewise calls them cells.

The following conditions of the crystalline lens may be observed in Mammalia. In a foetal pig, three and a half inches in length, the greater part of the fibres of the lens is already formed; a portion, however, is still incomplete; and there are many round cells awaiting their transformation. The perfected fibres form a sphere in the centre of the lens; but there is no laminated structure as yet perceptible in it. The fibres may readily be separated from each other, and proceed in an arched form from the anterior towards the posterior side of the lens. This sphere, composed of the perfected fibres, becomes surrounded, in the circumference of the lens, with a thick and broad zone of fibres, which are as yet imperfectly developed.