CHAPTER II 



THE CELLULAR BASIS OF HEREDITY AND 

 DEVELOPMENT 



A. INTRODUCTORY 



Heredity is to-day the central problem of 

 biology. This problem may be approached 

 from many sides, that of the observer, the 

 statistician, the practical breeder, the experi- 

 menter, the embryologist, the cytologist; but 

 these different aspects of the subject may be 

 reduced to three general methods of study, 

 (1) the observational and statistical, (2) the 

 experimental, (3) the cytological and embry- 

 ological. Before taking up these different as- 

 pects of heredity it is important that we should 

 have clear definitions of the terms employed 

 and a fairly accurate conception of the pro- 

 cesses involved. 



1. Definitions. — Heredity originally meant 



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