20 THE EVOLUTION THEORY 



follow that the lens is never liable to injury, and could not therefore 

 be adapted for regeneration. It can be bitten out along with the rest of 

 the eye by water-beetles or other enemies, and as far back as the time 

 of Bonnet and Bhimenbach (i 781) it was known that the eye of the 

 newt would renew itself if it were cut out, given that a small portion 

 of the bulb was left. But if this were removed the possibility of 

 regeneration was at an end. Thus, before the first artificial excision 

 of the lens, a regeneration-mechanism must have existed, by means of 

 which the eye with its lens was reconstructed, and this depends on 

 the characters of the cells of the eye itself — it is localized in the eye, 

 and without the presence of a piece of eye-tissue no regeneration can 

 take place. Is it then so especially remarkable that the lens should 

 be renewed when it is artificially removed without the rest of the 

 eyel The mechanism for its renewal is there, and is roused to 

 activity whether the lens alone or other parts of the eye also be 

 removed. We do not need, therefore, to assume the existence of 

 a pui-poseful or adaptive force ; it is more to the point to inquire 

 where the regeneration-mechanism which suggests this inference is to 

 be found. 



A definite answer to this is given in a detailed experimental 

 work recently published by Fischel. It corroborates what Wolff had 

 already found, that the substance of the new lens develops from 

 cells which cover the posterior surface of the iris, that is, from cells of 

 the retinal layer of the eye. First, the margin of the pupil begins to 

 react to the stimulus of the injury (extraction of the lens) ; its cells 

 enlarge, become clear, while previously they were filled with dark 

 pigment, and finally they proliferate. They thus form a cell-vesicle 

 similar to the ectoderm-vesicle from which the lens arises in the 

 embryo, and into this the already mentioned retina-cells from the 

 posterior wall of the iris grow, elongate, and arrange themselves to 

 form the so-called 'lens-fibres,' on whose form, arrangement, and 

 transparency the function of the lens depends. This is marvellous 

 enough, but not more marvellous than that a whole foot should grow 

 on the cut stump of a newt's leg, or that a whole eye should arise 

 from a residual fragment. Here, again, we do not know the processes 

 which cause the arrangement of the cells and their often manifold 

 locally-conditioned differentiations, in short, we do not know the 

 essential natu7'e of regeneration. But, in the meantime, we can 

 endeavour to find out which cell-groups regeneration is bound up with 

 in particular cases, so as to know where the vital particles, the ' deter- 

 minants,' which condition regeneration, are placed by nature. 



In this case there can be no doubt on that point : they are the 



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