136 CLELAND 



At the present time, cytologists are concerned with a great variety of 

 problems, so many that it will be possible to do no more than refer briefly 

 to two or three of the more important, upon which the limelight is focused at 

 the moment. 



One of the fundamental problems upon which marked emphasis is now 

 being placed relates to the nature of the gene. Mendel long ago showed that 

 heredity is based upon the existence of separate and distinct determiners 

 which are transmitted from parents to offspring through sperm and egg. The 

 characteristics of an individual depend upon what determiners of heredity 

 it receives from its parents. These determiners have become known as genes. 

 What, then, is a gene? What is it composed of? Where is it situated in the 

 cell? How does it produce its effects? How is it distributed from one genera- 

 tion to another? Do all cells in a body contain full sets of genes? If so, how 

 do genes multiply, so that the two daughter cells derived from a single cell 

 will each have all the genes which the parent cell had? Can genes change? 

 If so, what is the nature of these changes, and how are they accomplished? 

 Are genes discrete chemical entities? There are some biologists who claim that 

 they are not. 



It is obvious that the answers to these questions lie at least in part in the 

 area of cytology. Whatever their nature, genes are cellular entities. By a 

 combination of techniques from a number of relevant disciplines, much has 

 been learned about the gene already, and the major emphasis now being 

 placed upon this problem ensures that much more will be learned in the near 

 future. 



The first problem relating to the gene is its location in the cell and its cyto- 

 logical identification. Much progress has been made along this line in recent 

 years. Genetical methods have demonstrated the fact that, as a rule, genes 

 are duplicated every time a cell divides so that each daughter cell receives a 

 full set of the genes which the parent cell had. The geneticist has shown that 

 genes are associated in blocks, which tend to be inherited together, and they 

 have found that the genes are associated within each block in linear order, 

 like beads on a string, each gene having its particular position in the string 

 between specific neighbor genes. They have also found that genes may occa- 

 sionally change their position, and a variety of ways have been discovered 

 by which this can be accomplished. A section of the gene string in a given 

 block of genes can become inverted in position, the genes in this section 

 coming to lie in reverse order ; or a group of genes may be moved to a different 

 position in the block; or different blocks may exchange sections of their gene 

 strings. Sometimes genes seem to vanish completely, or they may be dupli- 

 cated so that a gene is represented more than once in a set of genes. All these 

 facts regarding the arrangement and order of the genes and possible rear- 

 rangements of this order have been detected by genetic means, but such 

 findings have not in themselves related the genes to any particular structure 



