xv] DIFFERENTIATION 253 



and remain complete after division. It seems evident that 

 only by some inherent polarity existing in all the parts of 

 the embryo can such a result be achieved, but whether such 

 polarity is or is not conceivable on a purely physico-chemi- 

 cal basis is a question too large to be discussed here. It 

 should be noted, however, that experiments on Echinoid 

 embryos and other forms showing analogous phenomena 

 provide only one of several arguments brought forward by 

 DRIESCH in support of his belief in an extra-physical (non- 

 material) factor in the development and behaviour of living 

 beings, although if this particular argument should be shown 

 to be non-valid, the others would be greatly weakened if not 

 completely destroyed. 



Although in Echinoderm embryos, and in those of several 

 other forms, the parts are equipotential during the first 

 cleavages that is, each can, under experimental conditions, 

 give rise to any part of the larva this is not true of the eggs 

 or early embryos of many other forms. In such Molluscs as 

 Dentalium the removal of part of the egg before segmenta- 

 tion causes the larva to be defective, and in Ctenophores, if 

 the first two blastomeres are separated, each produces only 

 a half-larva. Facts of this kind have been used to show that 

 DRIESCH is mistaken, but, as he shows quite clearly, this 

 objection is not valid. In any embryo, whether the parts 

 in the early stages are equipotential or not, a stage comes 

 sooner or later in which differentiation has gone so far that 

 any cell or group of cells is only able to produce certain 

 parts and not others; in Echinoderms, for example, when 

 the cells that will naturally form endoderm are sufficiently 

 differentiated, their removal results in the production of a 

 larva lacking the archenteron. And such eggs as those of 

 Molluscs or Ctenophores, in which each blastomere will 

 only produce those parts which naturally arise from it, 



