CELL DIVISION 63 



necessity of having a complete set of genes in each cell. Incidentally 

 it is the genes in the chromosomes which make the word duplication 

 preferable to division in describing the formation of two chromosomes 

 from one, for the genes may be single protein molecules. As such they 

 could not be divided and retain their identity; they could, however, be 

 duplicated. 



How the cells in different parts of a multicellular animal become and 

 do different things when they contain identical chromosomes and genes 

 is a question which must be postponed until embryonic development is 

 studied. The even greater importance of genes and chromosomes in 

 reproductive cells, and a different type of cell division which manipulates 

 the genes in germ cells, must likewise await the discussion of embryology. 



References 



Calkins, G. N. Biology of the Protozoa. Lea & Febiger. (Pp. 208-245, types of 

 division in unicellular organisms.) 



Dahlgren, U., and W. A. Kepner. A Textbook of the Principles of Animal His- 

 tology. The Macmillan Company. (Chap. V.) 



MiNCHiN, E. A. An Introduction to the Study of the Protozoa. E. J. Arnold & Son, 

 Ltd. (Chap. VII.) 



Sharp, L. W. An Introduction to Cytology. 3d Ed. McGraw-Hill Book Com- 

 pany, Inc. (Chap. VIII.) 



Wilson, E. B. The Cell in Development and Heredity. 3d Ed. The Macmillan 

 Company. (Chap. II.) 



