252 ROLE OF CYTOPLASM [CH. 



egg owe their distinctive colours to the presence of mito- 

 chondria, Golgi elements and a certain amount of yolk in 

 the different regions. It seems much more probable that 

 these formative substances are present in regions which in 

 normal development will give rise to parts where they are 

 needed, than that their presence in itself in those parts 

 should be the cause of specific differentiation. 



Further support for this idea of polarity independent of 

 visible substances may be derived from DRIESCH'S famous 

 experiments on Sea-urchin embryos. He finds that if, before 

 any great differentiation has taken place, the embryo is 

 divided in any plane, the fragments, if large enough, will 

 produce complete larvae. From this it follows that a cell 

 which in natural conditions would have formed part of one 

 organ, in the divided embryo becomes part of another. The 

 fate of a cell thus depends not only on its own inherent 

 properties, but also on its position with respect to other 

 surrounding cells, and this seems to involve the conclusion 

 that the larva as a whole has some sort of polarity which 

 to some extent at least is independent of the character of 

 the separate cells of which it is composed, and therefore 

 independent of any factors which may be borne specifically 

 by their nuclei. 



From facts of the kind just men tioned,DRiESCH has drawn 

 the conclusion that living organisms differ fundamentally 

 from all non-living matter in their activities, for while all 

 changes in non-living matter are mechanical, that is, depen- 

 dent solely on chemical and physical forces, he considers 

 that it is impossible for any mechanism to bring about the 

 results observed in living beings. A Sea-urchin embryo can 

 be divided in its early stages in any direction, and the parts, 

 if large enough, will produce complete larvae, and DRIES CH 

 maintains that no conceivable mechanism can be so divided 



