446 INTRODUCTION TO CYTOLOGY 



forth in all its significance the cytological explanation of the Mendelian phe- 

 nomena that is offered by the behavior of the chromosomes, and thus initiated 

 the remarkable movement in this direction that followed. 



It was in such a manner that cytology was brought into an alliance 

 with genetics. As in other instances in the history of science, the frontier 

 of one field of research turned out to be the border of a neighboring field, 

 and the gratifying yield of significant results along the line of meeting 

 has been due in no small measure to the fact that those who formerly 

 worked independently on the two sides can now employ their combined 

 methods and speak the same language. The field of physics has con- 

 tributed a valuable tool in the form of X-radiation. By means of this 

 tool workers in modern cytogenetics are now able to obtain desired data 

 far more rapidly than would be possible without it, and they are entitled 

 to hope that they may some day exercise a fuller measure of control over 

 the course of evolution by some such means. 



Closely associated with the movement outlined above has been 

 another line of investigation characteristic of the present century, namely, 

 the study of the morphology of the chromosomes and the comparison of 

 the chromosome complements of related organisms. Research in this 

 field was stimulated by the discovery of heteroploidy and observations on 

 chromosome behavior in hybrids early in the century (p. 339 et seq.), while 

 its problems were placed in a much clearer light by the work on the 

 morphology of individual chromosomes begun about 1910 by S. Nawas- 

 chin (1857-1930) and his associates. Such researches have already 

 yielded important evidence with regard to systematic relationships, par- 

 ticularly among plants, and they promise to contribute largely to the 

 solution of many outstanding problems of this nature. During 

 the years which lie ahead, one of the most fruitful regions in the 

 field of biology with its disappearing hues of subdivision should be 

 this cytotaxonomy. 



Of special importance also is the modern renewal of the investigation 

 of protoplasm, its inclusions, its differentiations, and its reactions in the 

 living state. This type of study has been greatly promoted not only by 

 the development of methods of tissue culture and micromanipulation 

 but also by the conception of protoplasm as a colloidal system. Investi- 

 gations in this field are removing many misconceptions growing out of the 

 too exclusive use of fixed material and are yielding partial explanations 

 for a variety of puzzling cytological phenomena. Moreover, they are 

 bringing cytology into ever more intimate associations with physical 

 chemistry, physiology, and experimental medicine, just as the study of 

 chromosomes has allied it with genetics and systematics. To such fields 

 cytology has made invaluable contributions; in return it has received 

 incentives to deal more directly and experimentally with major problems 



