PREFACE vii 



complexity, though a promising start in this direction has been made by 

 Mendelian analysis and its correlation with hypothetical " factors " 

 or " genes " in the idioplasm. Even when dealing with objects slightly 

 above the limits of visibility, the difficulties of observation are often very 

 great, so that two cytologists examining an identical object will often 

 give a different account of it. Many examples will occur to any cytologist 

 — for instance, the different accounts given of the changes undergone by 

 the chromosomes when passing into the resting nucleus at telophase. 

 The appearance of the disintegrating chromosomes of the same organism 

 has been variously interpreted as (i) vacuolation, (2) a splitting into 

 two threads, (3) the formation of a single spiral thread, (4) the formation 

 of two intertwined threads. 



Besides many other difficulties inherent in the nature of cytological 

 research, the science suffers especially severely from one of the difficulties 

 in the way of progress of all sciences. The researcher can only select 

 for study a minute fraction of the mass of objects presented to him, and 

 inevitably those objects appear to him significant, and therefore worthy 

 to be studied, which fit into his preconceived ideas. If a cytologist sets 

 out to study the gametogenesis of some animal, he will probably pass 

 under review through his microscope many hundreds of thousands of cells. 

 Out of these he can necessarily only select a minute proportion for detailed 

 study. The cells which he thus selects are, of course, those which seem 

 to him to represent stages in the process which he is endeavouring to 

 reconstruct. If he has already formed a theory regarding this process, 

 having a more definite mental image of the process as conceived by him 

 than of the possible alternatives he more readily picks out for study those 

 objects which appear to favour his theory than the others, which he 

 rejects (as he is bound to reject the great majority) as equivocal or of no 

 significance. This certainly appears to be the explanation of the partisan 

 nature of so much cytological work. 



The student must not get the impression from the above that it is 

 hopeless to discover the truth in cytology. Gradually one or other of 

 confficting views becomes recognized as being nearer the truth than its 

 rivals, or else some generally accepted principle is raised from the ashes 

 of them all — either by the gradual accumulation of evidence or by some 

 important discovery which is generally recognized as providin,^: the key 

 to the problem. In this way a science of cytology has grown up, firmly 

 estabHshed as regards its main outlines. 



