TRENDS OF THOUGHT 89 



continued to find peculiar and exceptional phenomena necessi- 

 tating modifications and additions to the previous theories, so 

 cytologists attempted to find their equivalents in cell structure 

 and behaviour. Thus arose the peculiar position in which genetic 

 theory outran cytological observation even to the limit of the 

 resolving power of the microscope. 



Once the universality of the ordered methods of nuclear 

 division, known as mitosis and meiosis, was established in 

 all the main groups of animals in which genetic investigation 

 had shown the Mendelian laws to operate, the cytologist's work in 

 this field was practically at a close. Not that many outstanding 

 problems did not remain, but, as subsequent work has shown, the 

 physical limitations of microscopical technique were too great 

 and the geneticist has had to proceed on assumptions. The 

 classical examj^le of this is, of course, " crossing over," which has 

 never been actually demonstrated cytologically, although a period 

 has been described in which it could occur. 



The present trend of cytology has arisen, too, from studies 

 originally pursued in other sciences. The rise of the hormone 

 theory and the importance of enzymes in relation to both sexual 

 and general physiological phenomena has stimulated investigations 

 into the structure and function of the various components of the 

 cell other than the nucleus. Thus most of the recent work centres 

 around the activities of the Golgi apparatus, mitochondria, the 

 phenomena of secretion, absorption of food and other activities of 

 the normal cell. It will be noticed, too, that although much of the 

 pioneer work in this direction was carried out on germ cells, whose 

 rhythm, function and ultimate fate were fairly well known, atten- 

 tion is now given to many of the other cells of the animal body. 



It is of course true that this tendency began more than ten 

 years ago, but much of this earlier work was isolated and un- 

 correlated and suffered in confusion as much from the want of 

 workers as from the lack of knowledge. The very considerable 

 progress made recently is chiefly due to improved technique. 

 This has occurred not only in improvements in method made 

 possible by advances in biochemistry and their application as 

 microchemical methods, but also in a general onslaught made upon 



