SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 385 



plified by the morphological differentiations of the parts of 

 organisms, we have here seen afresh exemplified in ways also 

 countless, by the physiological differentiations of their parts. 

 And in the one case as in the other, this change from uni- 

 formity to multiformity in organic aggregates, is caused, as 

 it is in all inorganic aggregates, by the necessary exposure 

 of their component parts to actions unlike in kind or quan- 

 tity or both. General proof of this is furnished by the order 

 in which the differences appear. If parts are rendered 

 physiologically heterogeneous by the heterogeneity of the in- 

 cident forces, then the earliest contrasts should be between 

 parts that are the most strongly contrasted in their relations 

 to incident forces; the next earliest contrasts should occur 

 where there are the next strongest contrasts in these relations ; 

 and so on. It turns out that they do so. 



Everywhere the differentiation of outside from inside 

 comes first. In the simplest plants the unlikeness of the 

 cell-wall to the cell-contents is the conspicuous trait of 

 structure. The contrasts seen in the simplest animals are 

 of the same kind: the film that covers a Ehizopod and the 

 more indurated coat of an Infusorian, are more unlike the 

 contained sarcode than the other parts of this are from one 

 another; and the tendency during the life of the animal is 

 for the unlikeness to become greater. What is true 



of Protophyta and Protozoa, is true of the germs of all organ- 

 isms up to the highest : the differentiation of outer from inner 

 is the first step. When the protoplasm of an Alga-cell has 

 broken up into the clusters of granules which are eventually 

 to become spores, each of these quickly acquires a mem- 

 branous coating; constituting an unlikeness between surface 

 and centre. Similarly with the ovule of every higher plant : 

 the mass of cells forming it, early exhibits an outside layer of 

 cells distinguished from the cells within. With animal-germs 

 it is the same. Be it in a ciliated gemmule, be it in the 

 unfertilized ova of Aphides and of the Cecidomyia, or be it in 



true ova, the primary differentiation conforms to the rela- 

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