SCIENTIFIC PROCEEDINGS. 



One hundred first meeting. 



Abstracts of Communications. 



Cornell University Medical College, October 15, ioip. 

 President Calkins in the chair, 



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The developmental stages at which mutations occur in the"germ 



tract. 



By Calvin B. Bridges (by invitation) . 

 [From the Zoological Laboratory, Columbia University.] 



In the fruit-fly Drosophila melanogaster about 300 primary 

 mutations have been found, most of which arose in cultures car- 

 ried on in the laboratory. A study of the critical cases among 

 these mutations has shown that a large majority of them origi- 

 nated at or very near the maturation stage ; that a few occurred in 

 the gonial cells some time prior to maturation; and that a few 

 occurred early in the segmentation stage. 



The conclusion that most mutations occur at the maturation 

 stage is based largely on the proportion of sex-linked recessives and 

 of dominants that have been first found as a single individual. 

 Approximately half of the sex-linked recessives have been dis- 

 covered as a single male. This is a surprisingly large proportion 

 and clearly means that in these cases the actual mutation occurred 

 in the mother, and at, or not more than a very few cell generations 

 before, the maturation of the egg. Those sex-linked recessives 

 that did not first appear as a single male have in the main appeared 

 as half the sons of a female already heterozygous for the gene. 

 In these cases the actual mutation had occurred at some inde- 

 terminate stage one or more generations previous to the appearance 



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