vw - 



HERITABLE DIFFERENCES 65 



Secondly, the mutation may occur in the fertilized 

 egg. This is zygotic mutation. In this case the change 

 is evident at once in the resulting individual since the 

 developing individual is the unfolding of what is pres- 

 ent in the zygote. Such a mutation, for example, oc- 

 curring after fertilization and not as the result of a 

 combination or cross, is reported in tobacco by Hayes 

 and Beinhart. 1 



Thirdly, in contrast to the two kinds of mutations 

 just described which, are distinctly germinal in origin, 

 there may be somatic mutations which fall directly 

 upon some individual somatic cell or tissue arising out 

 of the original germplasm and produce in turn such 

 abnormalities as "bud variations," chimaeras and the 

 like. In such a case all the cells and tissues arising 

 from the mutant somatic cell will express the mutation 

 and no others. Lehmann, 1920, proposes the term 

 metaclonosis for hereditary somatic modifications, re- 

 serving the term mutation for solely those instances 

 that involve a change in the genes. 



8. WHEN MUTATIONS_DCCUR 



It has been suggested by Standfuss that species 

 may go through the same kind of a life-cycle that indi- 

 viduals do, only taking infinitely more time to do it. 

 As shown in Figure 11, they are born of other species 

 and enter the prodigious growth period of infancy and 

 youth, both of which are characterized by much fluctua- 

 tion. With maturity they gradually become compara- 

 tively stable until the reproductive period is reached, 

 Science XXXIX, No. 992, p. 34. 



