28 Indications of Continued Formative Influence 



are sent out from this zone, they conduct themselves in 

 a quite indifferent way, though for a greater or less time 

 they preserve their germinative capacity potentially. 



While the experiments of Roux carried on carefully 

 and with astonishing exactness of observation, dem- 

 onstrate directly the continued remote and mediate action 

 which the formative part exercises upon the part being 

 formed throughout the whole of development, the exist- 

 ence of this action is confirmed by other investigations, 

 in a way which though indirect is not any less certain 

 on that account. 



They comprise all the cases in which the part removed 

 is regenerated by cells histologically different from those 

 of normal generation, for instance in which organs or 

 tissues of ectodermic, mesodermic, or entodermic origin 

 are reproduced in the regeneration by tissues having a 

 different blastodermic origin. 



It will suffice for our object to recall the most typical 

 example which has stirred up the two hostile camps of 

 the epigenesists and the preformists; we refer to that 

 of the regeneration of the lens in the eye of the tritons, 

 which, after its extirpation, is reproduced from the cells 

 of the iris, that is to say, from a material quite different 

 in character from that of which it is formed in normal 

 generation. 8 



The double epithelial layer of the iris, from the 

 marginal proliferation of which the new lens springs, 

 must exercise upon the lens in process of formation a 

 continuous action persisting throughout the development 



8 See e. g. Erik Miiller: Uber die Regeneration der Augenlinse 

 nach Exstirpation derselben beim Triton. Archiv f. mikrosk. Anat. 

 und Entwicklungsgesch. Band, XLVII. erstes Heft. Bonn. Cohen, 

 1896. P. 23 ff., especially 29 and 30. 



