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ORGANIC EVOLUTION 



under ordinary conditions have any descendants in the fol- 

 lowing generation. The whole body of the offspring comes 

 from two united germ cells, an egg from one parent and a 

 spermatozoon from the other parent. No bone cell, or muscle 

 cell, or any other body cell, in either parent, gives rise to any 

 part of the offspring. Weismann has used the term soma to 

 include all the cells of the body which are not germ cells, 

 that is, the muscle cells, bone cells, nerve cells, etc. . . . 

 This distinction between the germ cells, from which the 

 young are derived, and the soma cells, which ordinarily have 

 no offspring in the next generation but are destined to die, is 

 a very important one, and upon it must be based the discus- 

 sion of the inheritance of parental modifications. 



As a fertilized egg is developing into an adult organism 

 it divides into a number of portions called blastomeres, 

 certain of which will form the srerm cells of the new or^an- 



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ism, while the remainder will become its soma. The o-erm 



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cells of one generation are thus derived almost directly from 

 the germ cells of the preceding generation. 



The accompanying diagram may make the matter clearer. 



Generation A, 



Generation B, 



Generation C, 



Generation D, 



Germ cells 



Germ cells 



Germ cells 



Germ cells 



Soma. 



Soma. 



Soma. 



Soma. 



In the diagram the lines indicate lines of descent. Both the 

 germ cells and soma cells of any generation are derived from 

 the germ cells alone of the preceding generation. The 



