ON PHYSIOLOGICAL CHROMOMERES. 627 



types among the chromosomes has found a strong corroboration 

 in the study of the offspring of the mutants, known as semigigas. 

 These contain 21 chromosomes, one of their parental sets being 

 doubled. In the typical mutations, considered in this article, there 

 is almost always one chromosome in the same condition, the total 

 number being 15. Why this doubling should so constantly be asso- 

 ciated with them, we do not know, but it yields an easy means in 

 our discussions. We may consider a semigigas nucleus as one in 

 which the external conditions have provoked at once all of the 

 main changes which they produce singly in less extreme cases. If 

 this is true, the semigigas would contain the linkage groups for lata, 

 scintillans and all the others in the mutated or active condition, 

 and it would be sufficient to isolate the mutated chromosomes in 

 order to get at once the corresponding races. Now this is actually 

 what occurs after fecundation of the semigigas. In the reduction 

 division the chromosomes are exchanged according to chance. Part 

 of the offspring will get two or more mutated ones, and these will 

 yield mixed types, which may be left out of consideration here. 

 Another part will receive only one mutated rod each, and these 

 will be easily recognized in studying their nuclei. The expectation 

 will then be that in the latter category the seven main kinds of 

 typical mutants will be represented, and conversily this observation 

 will give as a direct a proof as is possible, of the nature of the mutable 

 chromomere in each of the seven chromosomes. 



The seven types of mutants found among the offspring of 0. 

 semigigas are, as a rule, those named above. They might be expected 

 to occur in equal numbers, but as yet the difficulties encountered 

 in these experiments are such as to occasion numerous deviations. 

 Much depends upon the pollen used and the conditions of the cul- 

 tures. In an experiment by Mr. Boedijn and myself the offspring 

 of a semigigas included 2.5% lata-\\kt specimens, 12.5% scintillans- 

 like, 22.5%cana-\\ke, \0% pallescens-Yike, \\.3%liquida-\\ke, 13.5% 

 spathulata-hke and 12.5% pulla, besides some rarer forms and this 

 shows these types to be the representatives of the main mutable 

 linkage groups in the seven different chromosomes. 



Summing up the results of this discussion we find that each of 

 the seven haploid chromosomes of CEnothera Lamarckiana must have 

 at least one linkage group or one physiological chromomere in a 

 mutable condition, while two of them have at least two such con- 

 stituent parts, viz. 0. mut. lata with albida and 0. mut. scintillans 

 with oblonga. 



