HEREDITY 



To these two principles we may now add 

 a third, viz.: (3) The individual consists 

 of two distinct parts : first, its body destined 

 to die and disintegrate after a certain length 

 of time; and, secondly, the germ-cells con- 

 tained within that body, capable of indefinite 

 existence in a suitable medium. 



The fertilized egg or zygote begins its in- 

 dependent existence by dividing into a number 

 of cells. These become specialized to form 

 the various parts and tissues of the body, 

 muscle, bone, nerve, etc., and by becoming thus 

 specialized they lose the power to produce any- 

 thing but their own particular kind of special- 

 ized tissue; they cannot reproduce the whole. 

 This function is retained only by certain un- 

 differentiated cells found in the reproductive 

 glands and known as germ-cells. They are 

 direct lineal descendants of the fertilized egg 

 itself. If they are destroyed the individual 

 loses the power of reproduction altogether. 



External influences which act upon the body 

 may of course modify it profoundly, but such 

 modifications are not transmitted through the 

 gametes, because the gametes are not derived 



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