208 



PHYSICAL BASIS OF HEEEDITY 



sibly, too, the irregular distribution of chromatin par- 

 ticles (chromidia) in the cytoplasm — aside from the 

 nuclear phenomena, or in connection with them — may 

 make the results similar in certain aspects to the distri- 

 bution of plastids in certain plant cells. 



Many species of plant lice — aphids — (Fig. 95, a) 

 propagate throughout the summer by parthenogenesis. 

 There is no chromosomal reduction during the develop- 

 ment of the egg. Each egg gives off only one polar body, 



$96 



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13 20 2a ii ^ II 



10 (g 5 16 li( ZD 13 



FiQ. 96. — Curve showing the non-effect of selection for the first twelve generations 

 for increase in body length, the heavy solid lines represent the fluctuations of the fraternal 

 means; the light solid line the fluctuations of the longest variant; the broken line the 

 fluctuation of the shortest variant. (After Ewing.) 



each chromosome splitting into two daughter chromo- 

 somes, so that the egg retains the whole number of chromo- 

 somes. Ewing has carried out an extensive experiment 

 with Aphis avence, selecting individuals through a num- 

 ber of generations for the length of the cornicles (honey- 

 dew tubes), for the length of the antennae, and for body 

 length. Considering here only the last, individuals were 

 selected for forty-four generations in a plus and in a 

 minus direction. The graph for the f ourty-fourth to the 

 sixty-third generation is shown in Fig. 96. The heavy 

 solid line represents the fluctuations of the longest vari- 



