208 



PHYSICAL BASIS OF HEREDITY 



sibly, too, the irregiilar distribution of chromatin par 

 tides (chromidia) in the c>i:oplasm — aside from th( 

 nuclear phenomena, or in connection with them — ma} 

 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 Qgg gives off only one polar body 



^'i vv VS H6 ^7 H'i H9 SO 51 51 '3 SH 55 56 57 51 59 60 61 Gl 63 



Fig. on. — Curve showing the non-effect of selection for the first twelve geneiatir iig 

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

 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 Qgg 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 antennas, 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 fourty-fourth to the 

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

 solid line represents the fluctuations of the longest vari- 



