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THE AMERICAN NATURALIST [ Vol. XLIX 



chromosomes in heredity not only would fill many pages, 

 bnt would expose numerous gaps in our present knowl- 

 edge, gaps that leave several important questions in the 

 balance. We shall assume frankly therefore that the 

 chromosomes are the bearers of the determiners of prac- 

 tically all of the hereditary characters that have been in- 

 vestigated by pedigree culture methods, acknowledging 

 freely our ignorance on many points, but maintaining that 

 while no facts have been discovered which offer insur- 

 mountable arguments against the viewpoint taken, the 

 following logical sequence of truths discovered at various 

 times and by different methods of research make a pretty 

 sound case upon which to base our practical conclusions. 



Relative Importance of Nucleus and Cytoplasm 

 There are several reasons far believing that of the two 

 parts of the cell, the nucleus and the cytoplasm, the former 

 plays the greater role in heredity. 



In general it is believed that the two parents contribute 

 equally in the production of offspring— that the male and 

 female contribution of potential characters is practically 

 the same. If there were a difference it would be shown 

 by divergent results in reciprocal crosses, but the investi- 

 gations following Mendel's method make it probable that 

 with the exception of sex and sex-linked characters, the 

 results of reciprocal crosses are generally alike. This 

 being true, it would appear that the principal basis of 

 inheritance must be sought elsewhere than in the cyto- 

 plasm, for in most observed cases the sperm is very much 

 smaller than the egg, and this difference is largely a dif- 

 ference in the amount of cytoplasm each carries. Is one 

 not to look for some significance in this disparity in size? 

 St ra .-burger, as well as other botanists, has even gone so 

 far as to declare the male generative cell in certain angio- 

 sperms to be simply a naked nucleus that slips out of its 

 cytoplasmic coat into the embryo sac, leaving the dis- 

 carded coat behind, and that stimuli proceeding from the 

 nucleus control the assimilation of food in the cell and 

 determine even the character of the cytoplasm itself. 



