cytology: the study of the cell 149 



from their behavior at this stage judge as to their similarity of structure and 

 hence their evolutionary relationship. By techniques of this sort, much has 

 been learned about the ancestry of plants important to our economy, such 

 as wheat and oats. 



From this very cursory and superficial account, it can be seen that cytology 

 is capable of throwing light upon many fundamental problems of biology. 

 It is especially concerned with the basic problems of protoplasm — its structure, 

 chemical composition, and behavior. It is also of importance in the solution 

 of problems in other areas of biology, such as taxonomy and evolution, plant 

 and animal breeding. It is not an exaggeration to say that there is no branch 

 of biology to which cytology does not contribute. It is even being used at 

 present to predict the sex of a human child before birth (Anon., 1956). 



Cytology demonstrates in unusual degree the essential oneness of living 

 nature. The average person is apt to be impressed more with the diversity 

 than with the unity of life. Plants seem so different from animals. Bacteria 

 seem so different from either plants or animals. The millions of different 

 species of animals and plants, the range from amoeba to man, from the 

 microscopic alga to the sequoia tree, are evidence of the ability of protoplasm 

 to take on myriad forms, to adopt multitudinous variations of structure with- 

 out losing that structural key which makes it alive. Truly the diversity of 

 living material is a profoundly impressive fact. And yet, it is also an amazing 

 fact that through all this diversity there runs a unity of structure and func- 

 tion which is equally impressive, and most of this unity reveals itself at the 

 cellular level. It is a striking fact that practically all organisms, plant or ani- 

 mal, follow the same laws of heredity, based upon the presence of genes car- 

 ried in chromosomes. These chromosomes divide and are transmitted, when 

 cells divide, to the daughter cells by the same process of mitosis and are par- 

 celed out to the individual reproductive cells by the same mechanism of 

 meiosis. Practically all cells in all organisms have mitochondria, and asso- 

 ciated with these are to be found the same respiratory enzymes and the 

 same complicated chemical cycles by which energy is liberated. Even the 

 green chlorophyll which seems to be such a distinctive and unique feature of 

 green plants is structurally very closely related to the heme molecule which 

 forms the colored part of the hemoglobin of the animal blood system. When 

 one descends to the cellular level, most of the differences which distinguish 

 one class of organism from another seem to disappear, and there remains a 

 common core of essential structure and function which appears to have been 

 developed very early in the evolution of life and to have been retained by all 

 forms of life as they have deviated from one another. The deviations have 

 involved relatively unessential aspects; the essential features of living proto- 

 plasm have been retained by all organisms — otherwise they would not have 

 been able to survive. 



This fact has very important practical applications. Cells have the same 



