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THE AMERICAN NATURALIST [Vol. XLIX 



fertilized as easily as they may be crossed with certain 

 particular individuals. 



3. The ease with which the ova of any animal "A" may 

 be fertilized by the sperm of other individuals may vary. 



Morgan (1913) concluded from his own work and that 

 of Adkins that there were wide differences in the compati- 

 bility of ova to different sperm. Fuchs (1914) maintained 

 that 100 per cent, of segmenting eggs can be obtained in 

 every cross if the ova are normal and a sufficiently con- 

 centrated sperm suspension is used. It is possible that 

 Fuchs is correct and that varying concentrations of sperm 

 suspension were the cause of Morgan's and Adkins 's re- 

 sults, yet the possibility of differences in this regard in- 

 herent in the individual is not to be overlooked. It will 

 be seen later that I regard the matter as of great impor- 

 tance to the general subject. 



4. A chemical basis for self-sterility is shown in Fuch's 

 experiments by (a) the decrease in ease of cross-fertiliza- 

 tion after contact of ova with sperm from the same ani- 

 mal, and by (b) the difference in ease of self-fertilization 

 after various artificial changes in the chemical equilibrium 

 of the medium surrounding the ova. 



From the botanical side various studies on the physiol- 

 ogy of self-sterility have appeared since such investiga- 

 tions were initiated by Hildebrand in 1866. At this time 

 it is necessary for us to consider only those of Jost (1907), 

 Correns (1912), and Compton (1913). 



Jost was able to show that in self-sterile plants tubes 

 formed from their own pollen were so limited in their 

 development that fertilization did not occur, although the 

 necessary length of pollen tube was easily developed after 

 a cross-fertilization. He saw as the cause of these phe- 

 nomena the presence of " individueller Stoffe." Pollen 

 was indifferent to "Individualstoff " of the same plant, 

 but was stimulated by that of other plants. 



Correns (1912), working with one of the bitter cresses, 

 Cardamine pratensis, obtained results to which he gave a 

 simpler interpretation. Starting with two plants, B and 



