in ORGANIC ADAPTABILITY 29 



form a small complete blastula, and the resulting gastrula and 

 Pluteus larva is a perfectly formed dwarf of only half (or quarter) 

 the normal size." l 



In Amphioxus the isolated blastomere cells produced 

 by division of the ovum segment from the beginning 

 like a smaller ovum. 2 Lastly if the gastrula of sea-urchin 

 or starfish be bisected either along the axis or at right 

 angles to it complete little organisms are obtained. 3 



One might say, looking at these cases alone, that the 

 embryo was determined to do its duty, and grow up true 

 to type, whatever happened. Or, dropping metaphor, one 

 might infer the existence of internal forces in the germ 

 cells absolutely determining the lines of growth, irre- 

 spectively of the material supplied to them. This infer- 

 ence, however, would not be altogether sound. Both re- 

 generation and development are largely conditioned by 

 external circumstances. Thus a Tubularia has the very 

 useful faculty of growing a new head, in case it should 

 have the misfortune to be decapitated. But this regenera- 

 tion can only go forward in water, and if the decapitated 

 end is buried in the sand, it is the other end which 

 develops the head. 4 Either end of the worm is able to 

 grow a head with the help of water, neither end without 

 it. 5 So again if growth were fixed by internal causes 



I alone, a tadpole must necessarily develop into a frog when 

 the time comes. But if at that point in its career a tad- 

 pole is prevented from leaving the water, it retains its tail 

 and gills while increasing in size. 6 Similarly the crus- 

 tacean Artemia salina assumes two specifically different 

 forms according as it is brought up in sea water or in 

 fresh water. 7 Thus while on the one side we find the 

 organism clinging, as it were, to its type, in spite of 



1 Wilson, op. cit. p. 308. 2 Ib. p. 309. 



3 Driesch, Science and Philosophy of the Organism, vol. I. p. 8. See 

 the whole of Part I. (especially A and B) for numerous illustrations and 

 a full discussion of the problem. 4 Wilson, op. cit. p. 325. 



5 Another remarkable point is that the " head " does not bud out from 

 the wound (and is therefore not technically regenerated) but can be 

 shown to be restored by the combined work of many parts of the stem. 

 Its position and size depend on the length of the section, and the growth 

 will occur at whatever point the stem is cut (Driesch, vol. I. p. 127). 



6 Verworn, p. 182. T Ib. p. 183. 



