THE 



AMERICAN NATURALIST 



Vol. L. April, 1916 No. 592 



THE MECHANISM OF CROSSING-OVER 

 HERMANN J. MULLER 

 Rice Institute 



It is the object of this paper to give an account of the 

 most important evidence thus far gained in regard to the 

 manner in which separation of linked factors— often inter- 

 preted as " crossing-over"— takes place, and to describe 

 an experiment in which a new method for studying the 

 occurrence of such separation is employed. This experi- 

 ment is still under way, but as it may be a considerable 

 time before the results are obtained in full, it would not be 

 advisable to withhold longer an account of this work and 

 of other work that bears on the nature of the " crossing- 

 over. ' ' 



I. The Discovery of Interchange between Homologoi^s 

 Chromosomes 



The question as to whether separation of linked factors 

 is due to pieces of homologous chromosomes changing 

 places with each other, carries us back to the question 

 whether the factors lie in the chromosomes at all. As is 

 well known, there is a large body of evidence from cy- 

 tology and from experimental embryology, showing that 

 the chomosomes are persistent, self-perpetuating struc- 

 tures which have a profound influence upon development. 

 But the first definite evidence that the Mendelian factors 

 are contained in the chromosomes lay in the striking cor- 

 respondence which was found between their respective 

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