S8 CRYSTALLINE LENS. 



mode of life lias 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 vege- 

 table, 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, w r e must 

 trace their development in the feet us. 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 charac- 

 teristic 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 

 pi. 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. Wer- 

 neck, who first observed them, likewise calls them cells. 



The following conditions of the crystalline lens may be ob- 

 served 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 tow r ards the posterior side of the lens. 

 This sphere, composed of the perfected fibres, becomes sur- 

 rounded, in the circumference of the lens, with a thick and 

 broad zone of fibres, which are as yet imperfectly developed. 



