

APPENDIX I 



INTERPRETATION 



Take the nucleus — what is it ? — apparently no more than a pellicle or 

 skin, a mere bladder containing fluid ! 



W. B. Hardy, 1925. 



The cytologist investigating chromosomes chiefly depends on the 

 observation of fixed and stained material. He has two means of judging 

 the value of particular preparations for his studies. The first is by the 

 inductive-deductive method. The effect of particular reagents and 

 treatments on particular structures is determined, laws of behaviour are 

 induced, and from these the value of any particular treatment deduced. 

 This method has been attempted earlier (by Fischer, Yamaha and 

 others), but it has never yet been practicable because the character of 

 the organisms treated, the methods of treatment, the reagents used, and 

 the objects pursued, are all so very diverse in relation to the amount of 

 comparative study that can be carried out. He therefore has to resort 

 to a second more empirical method as being of more immediate practical 

 importance. 



This method is the method of comparison of observations, and it 

 may be applied in three ways : — 



(i) The observations may be compared with those of living material 

 of the same organism at the same stage of development. This test has 

 been usefully applied in the resting nucleus (Schaede), in the prophase of 

 meiosis (PL XVI) and elsewhere, but it is necessarily of restricted use 

 with the ordinar}^ microscopic technique. 



The ultra-violet microscope has increased resolving power and 

 increased selectivity both in regard to focus and absorption. Thus 

 photographs of living cells taken by ultra-violet light have a sharper 

 definition and much sharper contrast at a higher magnification. 

 Chromosomes particularly have a strongly differential absorptive 

 capacity as compared with cytoplasm, not only during mitosis but — and 

 this may prove of the greatest significance in relation to the theory of 

 continuity — also during the resting stage. It has already been possible 

 to take photographs of living cells agreeing with those hitherto taken of 

 fixed and stained cells and in some respects equally instructive (PI. XVI). 

 This method evidently has an important future, especially perhaps in 

 the study of stages in the chromosome history of organisms which have 



563 



