THE GROWTH AND SCOPE OF BIOLOGY 



19 



follow the fundamental rule laid down by Mendel. His principles have 

 undergone some modification, as a result of the investigations of T. H. 

 Morgan and others, so that the known operations of heredity are no longer 

 so simple as Mendel's statement. Further complexities are still being 

 discovered, but with few exceptions they form a harmonious whole, and 

 genetics at the present time approaches more nearly the condition of an 

 exact science than any other division of biology. 



Fig. 14. — Gregor Johann Mendel, 1822-1884. {From a photograph taken about 1880. 

 Reproduced from the report of the Royal Horticidtural Society Conference on Genetics, 1906, 

 by permission of the President and Council.) 



Cytology. — The handmaiden of genetics in all this advance has been 

 the science of cytology, which deals with the very small structures of the 

 cell. Advance in this field beyond the stage to which Max Schultze 

 brought it has depended upon further improvement of the microscope, 

 the discovery of dyes or stains by which these minute objects could be 

 made more readily visible, and the invention of mechanical devices for 

 cutting cells into very thin sections. These improvements in technique 

 led early to an understanding of cell division (in the eighteen seventies) 

 and later of the ripening of the germ cells. While cytology has been 

 concerned with all sorts of cell structures, the chromosomes, minute 

 objects in the cell nucleus, have long been regarded as of chief importance. 

 It is the chromosomes that have allied cytology so closely with genetics, 

 for the machinery of heredity is found in the chromosomes. At first, in 

 this alliance of genetics and cytology, the latter took the lead. Chromo- 



