114 THK ^,KR^[-PI,ASM 



be affected secondarily by external intiuences. Cells moreover 

 exist, the idioplasm of which peri)iaiienlly retains the possi- 

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

 plants, which has already l^een mentioned, is likewise deter- 

 mined l)y tlie 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 l)ecomes 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 (^Hycfra). 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 Phylogexv 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 rci^oieratiim is not a priinaty quality of the organ- 

 ism, but tliat it is a pJtenoinetion 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 {loc. cit., p. 175), who, however, expresses him- 

 self very cautiously witli regard to this difficult subject as follows : — 'so 



