LECTURE XX 

 REGENERATION 



Budding and division — Every theory of regeneration in the meantime only 

 provisional, a mere 'portmanteau theory' — Regeneration not a primary character — 

 Volvox — Hydra— Vital affinities — Planarians^Heteromorphoses — Enemies of Hydroid- 

 colonies — Regeneration in Plants — In Amphibians — In Earthworms — Different degrees 

 of regenerative capacity according to the liability of the part to injury — Different results 

 of longitudinal halving in Earthworms and in Planarians — Regeneration in Birds — 

 The disappearance of the power of regeneration is very slow — Morgan's experiments 

 on Hermit-crabs— Autotomy in Crustaceans and Insects — Regeneration of the lens in 

 Triton. 



We have endeavoured to explain the handing on of the comple- 

 ment of heritable qualities from one generation to another as due to 

 a continuity of the germ -plasm, and we assumed that the germ-cells 

 never arise except from cells in the ' germ-track ' ; that is, from cells 

 ■which are equipped, from the fertilized egg-cell onwards, with a com- 

 plete sample of slumbering germ-plasm, and are thereby enabled 

 to become germ-cells, and, subsequently, new individuals, in which 

 the aggregate of inherited primary constituents implied in the germ- 

 plasm can again attain to development. 



We have now to consider other cases of inheritance in relation to 

 the same problem— the origin of their hereditary equipment. 



We know, of course, that new individuals may arise apart from 

 germ-cells, that, in many of the lower animals and in plants, they 

 may arise by budding and fission. 



For both these cases the germ-plasm theory will suffice, with 

 a somewhat modified form of the same assumption which we made in 

 regard to the formation of germ-cells. The origin of a new individual 

 by budding seems often, indeed, to proceed from any set of somatic 

 cells in the mother animal ; but somatic cells, if they contain solely 

 the determinants controlling themselves, cannot possibly give rise to 

 a complete new individual, since this presupposes the presence of all 

 the determinants of the species. But as these determinants cannot 

 be formed de novo, the budding cells must contain, in addition to the 

 usual controlling somatic determinants, idioplasm in a latent, inactive 

 state, which only becomes active under certain internal or external 

 influences, and then gives rise to the formation of a bud. The source 

 II. v> 



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