CHAPTER VII 



THE GERM CELLS AND THE PROCESSES OF 



DIFFERENTIATION, HEREDITY, AND 



SEX DETERMINATION 



THE problem of heredity is the problem of development. 

 The student of heredity is concerned primarily with the com- 

 parison of the traits of adult organisms as they appear in suc- 

 cessive generations, and with the methods of the distribution 

 of distinctively individual parental characteristics among 

 successive generations of offspring. The student of develop- 

 ment is concerned primarily with the genesis of the traits of 

 the individual, with that continuous and orderly sequence of 

 changes that gives to the single-celled zygote the final form of 

 the fully matured animal. 



It might seem, therefore, that any consideration of the 

 problems of heredity is somewhat out of place in an account 

 of the processes of development. At one time this might 

 justly have been urged. But to-day the students of heredity 

 and of embryology have in common much that is fundamental. 

 Their interests meet in the ideas that the organism is a specific 

 creature at every stage of its existence, from zygote to adult; 

 that the qualities of each later stage are conditioned by those 

 of an earlier; and so ultimately the structural and functional 

 differentiations of the adult must be traced back to correspond- 

 ing differentiations of the zygote, or even to pre-conjuga- 

 tion phases of the gametes. It is the common endeavor of 

 the students of embryology and of genetics to answer the 

 question why the egg of a star-fish develops into a star-fish, 

 with the characteristics of its parents, rather than into a sea- 

 urchin, although it may develop in the same dish of water 

 with other eggs that do develop into sea-urchins. 



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