THE CHROMOSOMES IN HEREDITY. 245 



from direct cytological study. A good example of the class 

 is the "mosaic" fruit of Datura obtained by Bateson and 

 Saunders, which, although in general exhibiting the thornless re- 

 cessive condition, showed in exceptional cases a thorny patch. Of 

 this case Bateson says : " Unless this is an original sport on the 

 part of the individual, such a phenomenon may be taken as indi- 

 cating that the germ-cells may also have been mosaic." I must 

 confess my failure to comprehend just what is here meant by 

 mosaic germ-cells. I have attempted to show that in all proba- 

 bility the germ-cells are normally a mosaic of maternal and 

 paternal chromosomes, but very evidently this is not Bateson's 



meaning. 



From the standpoint of the chromosome theory I would sug- 

 gest a possible explanation of the conditions as follows : We 

 have already assumed that the somatic chromosome group, 

 having a similar number of members to that of the cleavage 

 nucleus and derived from it by equation divisions, is made up in 

 the same way of pairs of homologous chromosomes. Every 

 somatic cell, by this conception, must contain a double basis in 

 the field of each character it is capable of expressing. In strictly 

 Mendelian cases one of the homologues is uniformly dominant 

 throughout the parts of the organism in which the character is 

 exhibited. As already noted, however, it is unlikely that all the 

 descendants of a dominant chromatin entity will be dominant. 

 This is shown by the experiment of de Vries with sugar beets, 

 which are normally biennial but always produce a small per- 

 centage of annual plants or " runners," which latter are regarded 

 as recessives. The percentage of these runners may be in- 

 creased by rearing the plants under unfavorable conditions and 

 this is taken as evidence that the recessive allelomorphs may 

 become dominant under such conditions. 1 



If each cell contains maternal and paternal potentialities in re- 

 gard to each character, and if dominance is not a common func- 

 tion of one of these, there is nothing to show why as a result of 

 some disturbing factor one body of chromatin may not be called 

 into activity in one group of cells and its homologue in another. 

 This would produce just the sort of a mosaic which Bateson and 



1 Cf. Bateson and Saunders, pp. 135, 136. 



