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THE AMERICAN NATURALIST [Vol. LI 



before they will develop, and always produce males. 

 Many other peculiarities in the behavior of chromosomes 

 that have been reported from time to time may also be 

 due to the influence of the environment (cytoplasm), and 

 there seems to be no reason why factors carried by the 

 chromosomes should not be affected by the cytoplasm as 

 well as are entire chromosomes. 



By the interaction of Mendelian factors with the cyto- 

 plasm during the germ-cell cycle, it is even possible to 

 explain the fact that crossing over" occurs in the 

 females of Drosophila, but not in the males.^^ In the 

 latter, the spermatocytes do not pass through a pro- 

 nounced growth period, and hence there is comparatively 

 little nucleo-cytoplasmic interaction, and since the cyto- 

 plasm carried by the sperm may be considered negligible, 

 the factors borne by its chromosomes are not interfered 

 with. In the female, however, there is ample opportunity 

 for such interaction during the growth period, and factors 

 at this time may be influenced by the cytoplasm or may 

 influence the cytoplasm in such a way as to cause an ir- 

 regular distribution of chromosomal factors. 



To the writer the following conclusions seem justified. 

 The insect egg at the time of maturation is a mosaic of 

 differentiated .cytoplasmic areas predetermined to de- 

 velop into definite parts of the embryo. This organiza- 

 tion has resulted from the interaction of nucleus and 

 cytoplasm during the germ-cell cycle. Such interaction 

 is taking place at all times, but is visible only when such 

 processes as the protrusion of chromidia or chromatin- 

 diminution occur. The many cases of cytoplasmic con- 

 trol over chromatin behavior, and the apparent failure of 

 the factors for the characters commonly used by geneti- 

 cists to influence the egg organization, indicate the im- 

 portance of more careful studies of the genesis of this 

 organization. The importance of such studies is empha-- 

 sized by the possibility that they may help toward the 

 solution of the problem of the method of evolution. 



51 Morgan, Sturtevant, Muller, and Bridges, 1915, "Mechanism of Men- 

 delian Heredity." 



