CHAPTEK X. 



HEREDITY. 



A most essential factor in evolntion is heredity, or the 

 transmission of parental physical and mental character- 

 istics to the offspring. 



The medium or physical basis of heredity must, reason- 

 ing by exclusion, be pi'esent in the sexual cells, i.e., the 

 egg and the male or sperm cell. 



Now, since the spermatozoon represents the nucleus of 

 the egg, it follows that a certain portion of the nucleus is 

 the medium or " bearer of heredity," and since the egg- 

 nucleus and sperm-nucleus contain the same quantity of 

 chromosomes or bits of chromatin to the cleavage-spin- 

 dles (Figs. ?>a, oil), the chromatin is supposed to be the 

 bearer of heredity. 



Jaeger in 1876 first suggested that " through a great 

 series of generations the germinal protoplasm retains its 

 specific properties, dividing in every reproduction into 

 an ontogenetic portion, out of which the individual is built 

 up, and a pliylogenetic portion, which is reserved to form 

 the reproductive material of the mature offspring. This 

 reservation of the pliylogenetic material I described as the 

 continuity of the germ-jirotoplasm." 



Weismann in 1885 expanded this view, and substituted 

 for Darwin's hypothesis of " pangenesis " the now famous 

 doctrine of "the continuity of the germ-plasm." This germ- 

 plasm (consisting of particles of chromatin) is, as the result 

 of the fertilization of the egg, "continuous from one gen- 

 eration to another in an unending succession, and from it 

 the germ-cells of each generation are produced"; thus 



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