114 1HK GERM-PLASM 



be affected secondarily l)y external intiuences. Cells moreover 

 exist, the idioplasm of which permanently retains the possi- 

 bility of development along one of two lines. "Dichogeny' in 

 plants, wliich has already been mentioned, is likewise deter- 

 mined by the idioplasm, inasmuch as the latter must contain two 

 kinds of determinants, one or the other of which either remains 

 inactive owing to the nature of the external intiuences acting 

 upon the cell, or else becomes active and determines the cell. 



There are, however, no such things as 'embryonic cells' in 

 the sense in which this term is used by authors. In the fresh- 

 water polype {Hydra), for instance, cells which are young 

 and histologically undifferentiated — the so-called -interstitial 

 cells' — are present in the deeper part of the ectoderm: these 

 can certainly give rise to various structures, viz., to ordinary 

 ectoderm-cells, nettle-cells, muscle-cells, sexual-cells, and in all 

 probability to nerve-cells also. It would nevertheless be absurd 

 to suppose that any particular interstitial cell is capable of 

 developing into any one of these structures. It obviously con- 

 tains either germ-plasm, i.e., the whole of the determinants, — in 

 which case it can develop into a sexual cell, — or only the deter- 

 minants of a thread cell or of one of the other kinds of cells, and 

 then it can only give rise to one of the corresponding structures, 

 and can never develop into a sexual cell. 



2. The Phvlogeny of Regeneration 



It may, I believe, be deduced with certainty from those phe- 

 nomena of regeneration with which we are acquainted, that the 

 capacity for regeneration is not a pri)nary quality of the organ- 

 isnif but that it is a phefionienon of adaptation . 

 • The power of regeneration has hitherto been practically always 

 regarded as a primary quality of the organism, — that is to say, 

 as a direct result of its organisation : it has been looked upon 

 as a faculty for which no special arrangements are required, 

 but which naturally results as an unintentional secondary effect 

 of the organisation which exists independently of it. 



This view is based on the idea, which is in general a correct 

 one, that the regenerative power of an animal is inversely 

 proportional to its degree of organisation.* If this were univer- 



* Cf. Herbert Spencer Hoc. cU., p. 175), who, however, expresses him- 

 self very cautiously with regard to this ditftcult subject as follows : — 'so 



