OUTLOOK 



material has touched the cortical layer. Clearly these events must all 

 be interrelated, though we yet know nothing of how this is achieved. 



The attempt is often made to bridge such gaps by a broad appeal to 

 theory and generalization. It may not be amiss briefly to consider this 

 tendency. It is difficult to resist the impression that the elaboration of 

 theories in cytology is sometimes carried to great lengths. Darlington,^ 

 for instance, in his 'balance theory of mitosis' gives a graphical 

 representation of the charges with which he endows centrosomes and 

 centromeres during mitosis and meiosis, though without an actual scale 

 in millivolts. Within recent years, the nucleic acids have proved a 

 favourable subject for exercises of this kind, the scope of which has not 

 been restricted to this planet alone. The question at issue is not whether 

 truths beyond the immediate reach of observation can be apprehended 

 by imaginative inference, but whether such theories have been found 

 to serve as a useful basis for further research, and to stimulate fresh 

 inquiry into particular aspects of cell division. It is doubtful whether a 

 survey of the progress of cytology would uphold such a claim, though 

 it might be urged that it is the experimenter who has subsequently 

 failed to play his part in testing these hypotheses. In practice, how- 

 ever, the wider is their scope, the less readily can they be tested by a 

 specific inquiry. Anything in the cell could be explained by a general 

 appeal to the nucleic acids, as readily before an experiment as after it. 



The generalizing impulse is often expressed in another way. There is 

 a strong tendency to assume that observations made in a specially 

 favourable material must be everywhere valid in virtue of their clarity 

 and precision. If there are discordant results elsewhere, they can be 

 brushed aside as unimportant exceptions. Examples of this are by no 

 means uncommon; two may perhaps be quoted. Cleveland^ in des- 

 cribing some of his very remarkable observations on the cytology of the 

 flagellates parasitic on termites, claims that they apply also to cellular 

 organisms : 'The close similarity between the behaviour of t^ese hyper- 

 mastigote centrioles, and the centrioles of other cells leaves no room to 

 doubt the general application of the observations on these flagellates to 

 mitosis in both animals and plants,' 



Again, Loveless and Revell^ in surveying methods of research 

 on the effects of mutagenic chemicals make the statement : 



By the use of the precise techniques that had previously been devised for the 

 detection and quantitative estimation of X-ray effects, they (geneticists and cyto- 

 logists) have shown that certain classes of these compounds are mutagenetic agents, 

 for they can cause heritable nuclear changes that are demonstrably similar to 

 those induced by X-rays. Consequently they have used as experimental materials 

 those, such as Drosophila and the classical cytological objects, which were best 

 suited to the application of these techniques, and they have felt justified in doing 

 this by their confidence that their results could in general be extrapolated to other 

 cellular systems. 



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