Heredity and the Origin of Variations. 127 



either (1) in those cells at or adjoining the mutilated 

 surface which are concerned in the regrowth of the lost 

 part ; or (2) in the general mass of cells of the mutilated 

 organism. 



There are difficulties in either view. Professor Sollas, 

 supporting the former, says,* " This power [in the snail] 

 of growing afresh so complex and specialized an organ as 

 an eye is certainly, at first sight, not a little astonishing, 

 but it appears to be capable of a very simple explanation. 

 The cells terminating the cut stump of the tentacle are the 

 ancestors of those which are removed; a fresh series of 

 descendants are derived from them, similarly related to the 

 ancestral cells as their predecessors which they replace ; 

 the first generation of descendants become in turn ancestors 

 to a second generation, similarly related to them as were 

 the second tier of extirpated cells ; and this process of 

 descent being repeated, the completed organ will at length 

 be rebuilt." This explanation is, however, misleading in 

 its simplicity. The cells terminating the cut stump are 

 not the direct ancestors of those which are removed, except 

 in the same sense as gorillas are ancestors of men. They 

 are rather collateral descendants of common ancestors. I 

 think that Professor Sollas would probably agree that, 

 though the lens and " retina " are of epiblastic (outer 

 layer) origin, their relationship with the epiblastic cells at 

 the cut stump is a somewhat distant one. In the repro- 

 duction of the lens the cell-heredity is not direct, but 

 markedly indirect. And it is somewhat difficult to under- 

 stand by what means the ordinary epiblastic cells of the 

 cut stump, which have had no part in the special and 

 peculiar work of lens-production, should be enabled to 

 produce cell-offspring, some of which, and those in a 

 special relation to other deeper-lying cells, possess this 

 peculiar power. 



On the other hand, if we turn to the view that the 

 reproduction is effected, not by the cells of the cut surface 

 alone, but by the general mass of cells in the mutilated 



* Nature, vol. xxxix. p. 486. 



