THE GERM-LAYERS IN REGENERATION 2O? 



not affect seriously the argument in fact, it makes the case all the 

 more remarkable if these imperfect attempts are in the direction of 

 useful responses. Fischel sums up his conclusions as follows : " It is 

 not necessary, and it is irreconcilable with the facts, to describe the 

 formation of the lens in a teleological sense, and to bring this case 

 forward as a proof of the universal application of a teleological 

 principle. As has been already stated, the facts in regard to this 

 case show much more clearly that the organism reacts to each 

 change always in a manner that corresponds to its limited possibili- 

 ties without regard to a teleological principle. A planarian, for in- 

 stance, responds to a stimulus and makes a new head, even when 

 it possesses one or more already ; a tubularian produces a hydranth 

 at its basal end, if this end is freely surrounded by water ; an actin- 

 ian forms a new mouth on the side of its body, etc. ; so also do the 

 cells of the pars ciliaris, and the pars iridica retince differentiate into 

 lens fibres. Working blindly, without respect to the consequences 

 as far as they concern the whole, the one thing only is pro- 

 duced for which the conditions are present that bring about its 

 formation in the cells." 



THE PART PLAYED BY THE " GERM-LAYERS" IN REGENERATION 



Our examination of the origin of the tissues and organs in the 

 new parts has shown that in most cases the old tissues give rise to 

 the same kind of tissue in the new part; or in some other cases, 

 as in the nervous system, the regenerating organs arise from the 

 same " layer " as that from which they develop in the embryo. 

 These facts have led many writers to state that the tissues and 

 organs in the regenerated part arise from the same germ-layers as 

 do the same parts in the embryo. It is supposed that ectoderm 

 gives rise to ectoderm, and to those structures that arise from the 

 ectoderm in the embryo, as, for instance, the nervous system, stomo- 

 daeum, etc. The endoderm is supposed to give rise to endoderm, and 

 to endodermal structures, and the mesoderm to mesoderm and its 

 derivates. So fixed has this opinion become that it is not uncom- 

 mon to find investigators proclaiming the triumphant success of their 

 results, because they have been able to trace the organs in the regen- 

 erated part to the same germ-layers that give rise to these organs in 

 the embryo. Before deciding as to the value of this point of view, 

 let us examine briefly the foundations of the so-called germ-layer 

 hypothesis. 



The origin of this hypothesis goes back at least to 1/59, when 

 . F. Wolff maintained his thesis that the digestive tract of the chick 

 exists as a flat, leaf-like structure that subsequently rolls up into 



