80 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



single cells of a blastula of Echinus is the same for all of 

 them ; their prospective value is as far as possible from 

 being constant. 



But we may say even a little more : what actually will 

 happen in each of the blastula cells in any special case of 

 development experimentally determined depends on the 

 position of that cell in the whole, if the " whole ' is put 

 into relation with any fixed system of co-ordinates ; or more 

 shortly, " the prospective value of any blastula cell is a 

 function of its position in the whole." 



I know from former experience that this statement wants 

 a few words of explanation. The word " function " is em- 

 ployed here in the most general, mathematical sense, simply 

 to express that the prospective value, the actual fate of a 

 cell, will change, whenever its position in the whole is 

 different. 1 The " whole 5: may be related to any three 

 axes drawn through the normal undisturbed egg, on the 

 hypothesis that there exists a primary polarity and 

 bilaterality of the germ ; the axes which determine this sort 

 of symmetry may, of course, conveniently be taken as * 

 co-ordinates ; but that is not necessary. 



The Potencies of Elementary Organs in General 



Before dealing with other very young germs', I think it 

 advisable to describe first an experiment which is carried 

 out at a later stage of our well-known form. This experi- 

 ment will easily lead to a few new concepts, which we 

 shall want later on, and will serve, on the other hand, as a 



1 A change of the position of the cell is of course effected by each variation 

 of the direction of the cut, which is purely a matter of chance. 



