CHAPTER XXVI 

 HISTORICAL SKETCH 



Cytology is about a century old. Before protoplasm had come to be 

 recognized as the physical basis of life, and the cell as essentially a proto- 

 plasmic unit, cytology as a distinct branch of biology scarcely existed, 

 although important investigations of the structure and development of 

 plants and animals had been in progress for many years. The course 

 followed later by the growing science was so profoundly affected by some 

 of these early investigations that they will be set forth below as a part 

 of the background of our subject. It will be seen that the discovery of the 

 cellular organization of most organisms, and the formulation of an influ- 

 ential theory based on the cell as a unit of cardinal importance in develop- 

 ment, long antedated a proper conception of the true significance of 

 protoplasm. As a result, cytology has not only derived its name from 

 the cell, but it has been dominated by the cell concept from the first, 

 notwithstanding the fact that many of the fundamental problems 

 confronting the cytologist are presented by organisms which do not have 

 what is ordinarily regarded as cellular organization. 



The investigation of such protoplasmic structures as nuclei and 

 plastids was begun a century ago, long before their constitutional rela- 

 tionship was suspected, but most of our knowledge of these and other 

 kindred elements has been acquired during the past 50 years. The 

 last quarter of the nineteenth century witnessed an intensive study of 

 the behavior of cells, nuclei, and chromosomes in the various processes 

 involved in the growth and differentiation of the organism. Among the 

 noteworthy results of this study w^as the theory that the phenomena of 

 inheritance are in some way closely dependent on the activities of the 

 nucleus, a theory which received striking support after the principles 

 of Mendelian heredity were rediscovered in 1900. This rediscovery has 

 very largely determined the character of cytological research so far in 

 the present century. It has brought cytology into intimate association 

 with genetics and taxonomy, while a renewed study of living protoplasm 

 has resulted in similar alliances with physiology and biochemistry. One 

 of the most conspicuous and encouraging tendencies of the present day is 

 this closer correlation of the various subdivisions of biology. 



The Discovery of the Cellular Organization of Plants and Animals. — 

 A perusal of Aristotle's De Partibus Animalium and the Historia Plan- 

 tarum of Theophrastus serves to show that the ancients had considerable 



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