president's address. 27 



may be used to build a whole body or half-body, according to the 

 grouping they assume. After the first cleavage takes place, each 

 blastomere is set, as it were, for a half-development, but not so 

 firmly that a re-arrangement is excluded. It is through the 

 interpretation of facts of this kind that Wilson believes that we 

 can " reconcile the theories of cytoplasmic localisation and mosaic 

 development with the hypothesis of cytoplasmic isotropy. 

 Primarily the egg-cytoplasm is isotropic in the sense that 

 its various regions stand in no fixed and necessary relation 

 with the parts to which they respectively give rise. Secondarily, 

 however, it may undergo differentiations through which it acquires 

 a definite regional predetermination, which becomes ever more 

 firmly established as development advances. This process does 

 not, however, begin at the same time, or proceed at the same rate 

 in all eggs. Hence the eggs of different animals may vary widely 

 in this regard, at the time cleavage begins, and hence may differ 

 as widely in their power of response to changed conditions." 



For our present purpose the importance of the facts quoted 

 lies in their testimony to the general fact of an ultra-microscopical 

 organised structure of germ cells, which embodies and subserves 

 the intracellular expi'essions of living activity, just in the same way 

 as the visible bodily organs embody the more obvious and familiar 

 aspects of bodily function. 



It must therefore be maintained that neither the obscurity of 

 the problem of heredity, nor the leadings of the extraordinarily 

 striking phenomena of regeneration can be regarded as absolutely 

 incapable of being brought into line with other biological facts as 

 causally determined in the mechanical sense, far as Ave are at 

 present from any such achievement. 



And in the present connection it cannot be admitted that we 

 are under any sort of compulsion to abandon the natural-historical 

 interpretation of homology — the true guiding hypothesis of 

 modern morphology — simply because we cannot eff'ect a definitive 

 analysis of its more important factors. Inability to do this does 

 not, for example, deprive me of the solid conviction that the 

 morphological relationship existing between, say, the presence of a 



