320 THE PHILOSOPHY OF BIOLOGY 



chemistry, and they have yet to be supported by 

 experiment before we can accept them as a description 

 of what is to be observed in the processes of nuclear 

 division and segmentation. Further, it is certainly 

 the case that any one cell of the early embryo can give 

 rise to any part of the larva. The segmented embryo 

 is therefore a system of parts, all of which are potentially 

 similar to each other. But actually each of these parts 

 has a different fate in the process of the development 

 of the larva, and this fate depends on what is the fate 

 of the adjacent cells. There is also a plan or design 

 in the development of the embryo that is, a very 

 definite structure results from this process and each 

 of the cells shares in the evolution of this design. The 

 system of cells is therefore an harmonious equipotential 

 system. The cells themselves are not the ultimate 

 parts of this system, for each of them is an aggregate of 

 a very great number of substances which are physico- 

 chemically characterised at least our methods of 

 analysis seem to show that each cell is a mixture of a 

 number of chemical compounds, but we must never 

 forget that it is the dead cell which we thus subject to 

 analysis, and not a living organism. Let us call these 

 supposed chemical constituents of the living cells the 

 elements of the system ; then at the beginning of the 

 process of development the latter is composed of 

 elements which are not definitely arranged but which 

 are distributed in an ' homogeneous ' manner very 

 like the distribution which is effected on shuffling a 

 pack of cards. But as differentiation proceeds, the 

 elements of this system become unequally distributed, 

 and the diversity becomes greater and greater, attain- 

 ing its maximum when the definitive tissues and organs 

 of the adult become established, just as at the close of 

 a game of bridge the cards acquire a particular arrange- 



