REGENERATION 33 



found among many animals which occur only as 'persons' and not 

 as colonies or stocks, but only in those which are I'eadily liable to 

 injur}'', and only in the manner conditioned by their injury. In the 

 higher Metazoa the regenerative power becomes more and more limited, 

 and in the Mammals it sinks to a mere closing up of wounds. 



If we take a survey of the assumptions we have been compelled 

 to make from the standpoint of the theory to explain the develop- 

 ment of germ -cells, budding, and regeneration, it would seem as if it 

 were contradictory to assume that, on the one hand, complete germ- 

 plasm should be given to certain cell-series as inactive accessory idio- 

 plasm, and, on the other, that very numerous cells, at least in the lower 

 Metazoa, should have received the idioplasm of budding, and still more 

 numerous cells that of regeneration. But it is obvious that among the 

 lower Metazoa the idioplasm of budding and the idioplasm of regenera- 

 tion are equivalent; the same idioplasm, which, when liberated by 

 stimuli unknown to us, co-operates from two or three germinal layers 

 in the formation of a bud, effects, in response to the known stimulus of 

 injury, the regeneration of the nmtilated part. But germ-cells can never 

 arise in the Metazoa from the partial budding-idioplasm or regeneration- 

 idioplasm, because this is not complete germ-plasm, and because it can 

 only give rise to budding or regeneration through the co-operation of 

 two or more kinds of cells, while germ-cells always originate from one 

 cell and never arise from the fusion of cells. Germ-cells can thus only 

 arise from the cells of the germ -track, and in no other way, no matter 

 whether the germ-track lie in the ectoderm, as in the Hydromedusas, or 

 in the endoderm, as in true jell^^fishes (Acaleph^) and the Ctenophores, 

 or in the mesoderm, as in many higher groups of animals. It is only 

 apparently that these cells belong to one particular layer, for in reality 

 they are unique in kind, and they are simply assisted in their develop- 

 ment by one or other cell-layer, from which they not infrequently 

 emancipate themselves, as happens so notably in the Hydromedusaj. 

 As we have already said, it is only among plants that we must 

 think of budding as arising from cells which contain complete germ- 

 plasm, for here there are no ' germinal layers ' corresponding to those 

 of animal development, and the cells of * the growing point ' must be 

 equipped with the complete germ-plasm. The plant, like the Hydroid 

 stock and the Siphonophore colony, is saved from death, in spite of the 

 frequent loss of its members, mainly by the fact that it is capable of 

 producing, at almost any part above the ground, Ijuds which develop 

 into new shoots, with leaves and the like. This makes a power of 

 regeneration on the part of the individual leaves and flower-parts 

 superfluous, but at the same time it implies that an enormous number 



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