Cleland • Cytology: the Study of the Cell 



231 



ganisms and their reactions to environ- 

 ment. When one has analyzed these 

 activities he is still confronted with the 

 question as to the basic causes of these 

 phenomena and is inevitablv forced 

 back to the protoplasm itself— to its 

 chemical and physical properties, to 

 the way in which the cell and its con- 

 stituent parts behave, to the question 

 as to what parts of the cell initiate the 

 processes which eventuate in physio- 

 logical activit}', how these essential 

 ingredients in the cell maintain and re- 

 produce themselves and become dis- 

 tributed to the daughter cells when a 

 parent cell divides. Cytolog)' has thus 

 found itself concerned with the most 

 fundamental questions which a biolo- 

 gist can ask. 



A modern cytologist, therefore, 

 must be a man of parts— a broadly 

 trained person— ideally a biologist with 

 chemical, physical, mathematical and 

 statistical competence. Many eminent 

 cvtologists have utilized a wide variety 

 of materials, plant and animal, in the 

 course of their experiments. 



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 exist- 

 ence of separate and distinct de- 

 terminers which are transmitted from 

 parents to offspring through sperm and 

 egg. The characteristics of an individ- 

 ual 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 

 generation 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 thev accom- 

 plished? Are genes discrete chemical 

 entities? There are some biologists who 

 claim that thev arc not. 



The first problem relating to the 

 gene is its location in the cell and its 

 cytological identification. Much prog- 

 ress has been made along this line in 

 recent years. Gcnctical methods have 

 demonstrated the fact that, as a rule, 

 genes are duplicated even- time a cell 

 divides so that each daughter cell re- 

 ceives 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 to- 

 gether, and thev 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 occasionally change their position 

 and a variety of ways have been dis- 

 covered by which this can be accom- 

 plished. A section of the gene string in 

 a given block of genes can become in- 

 verted 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 duplicated so that a gene is repre- 

 sented more than once in a set of genes. 

 All these facts regarding the arrange- 

 ment and order of the genes and pos- 

 sible rearrangements of this order have 

 been detected by genetic means, but 

 such findings have not in themselves 

 related the genes to any particular 

 structure or region in the cell as ob- 

 ser\ed microscopically. Tliis has been 

 accomplished by cytological investiga- 

 tion. When Mendel's paper was dis- 

 covered in 1900 and the laws of the dis- 

 tribution of hcreditar\' determiners 

 were thus brought to light, the cvtolo- 

 gists found that they had already seen 



