6 FUNDAMENTALS OF CYTOLOGY 



machines, or microtomes, yielded perfect and uniformly thin sections 

 at relatively high speed. All these aids were welcomed by cytologists 

 and histologists whose work they greatlj' facilitated, though they too 

 often had the undesirable effect of diverting attention from living 

 material. 



Compound microscopes were brought to a very high level of efficiency 

 during the nineteenth century. During the classic period, apochromatic 

 objectives, highly corrected against chromatic and spherical aberrations 

 and with high resolving power, were perfected, and these, employed 

 with newl}^ devised eyepieces and condensers, furnished the cytologist 

 with the finest images of highl}^ magnified objects he had yet seen. 



The closing years of the century also saw the formulation of theories 

 that gave direction to many of the investigations yet to come. For 

 example, there were biologists, notably August Weismann, who pro- 

 pounded theories of the mechanism of individual development, heredity, 

 and evolution largely on the basis of what had recently been learned 

 about the behavior of cells, nuclei, and chromosomes throughout the 

 life cycles of organisms. Some of the concepts embodied in Weismann 's 

 theory of inheritance continued as a part of the framework of our modern 

 theory, although his theory of development has been abandoned. In 

 the field of cellular physiology the theoretical interpretations and laws 

 pertaining to solutions, which grew out of researches on osmotic pressure 

 and permeability in cells and nonliving systems, also continue as strong 

 influences in the present century. 



By way of summary we may say that the nineteenth century con- 

 tributed to the twentieth a number of fundamental observations, certain 

 intriguing problems, an array of useful techniques, and certain suggestive 

 guiding theories. Observation, technical skill, and theory are all 

 required in scientific research, and progress is surest when all three are 

 utilized in the correct proportions. 



Cytology in the Twentieth Century. — Cytology in the present 

 century is fortunate in having several new and extremely valuable 

 technical aids. Methods for the successful cultivation of living animal 

 and plant tissues under controlled conditions in glass containers have 

 been devised. For instance, there are living in flasks today (1942) 

 healthy cells which are the descendants of similar cells removed from the 

 heart of a chick embryo in 1912 (Fig. 3). Such cultures enable the 

 worker to learn many things concerning the capacities of cells in a direct 

 manner rather than by a series of inferences from fixed preparations or 

 even from similar cells living in the complex environment within the 

 body. Direct study is also facilitated by the micromanipulator, with 

 which one can dissect or inject normal living cells under the best high- 

 powered objectives (Fig. 4). Striking permanent records of observations 



