. The physical basis of heredity . Heredity. 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, V3 11 IS IS 1)7 li 19 SO 51 SI ?3 SI SS 56 S7 St S9.
. The physical basis of heredity . Heredity. 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, V3 11 IS IS 1)7 li 19 SO 51 SI ?3 SI SS 56 S7 St S9. ^ 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 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-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Morgan, Thomas Hunt, 1866-1945. Philadelphia : Lippincott
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