392 University of California Publications in Agricultural Sciences [Vol. 2 



malities in chromosome organization, not to mention duplication of the 

 ends of the chromosomes. One must conclude, therefore, that either 

 there are no duplicational secondaries in Crepis or their nature should 

 be explained in some other way. It is hoped that further genetic 

 study on trisomies will throw light on this question. 



The frequencies of different trisomic types is not the same. In 

 C. capillaris apparently the triplo-D type (possessing one satellited 

 chromosome extra) is the most common one, while the triplo-A type 

 occurs less frequently. Finally, the third simple trisomic type, triplo- 

 C, probably occurs extremely rarely, if at all ; at least, it was not 

 found although numerous trisomies of the other two types were dis- 

 covered. Consequently, C capillaris exhibits some analogy in this 

 respect to Drosophila melanogaster; for according to Bridges (1923) 

 individuals with extra II or III chromosomes do not occur in the 

 progeny of triploid females but triplo-X and triplo-IV flies are fairly 

 common. The lack of these combinations in Drosophila is reasonably 

 explained bj^ the disturbances produced by the unbalanced excess 

 of large chromosomes ; in the case of Crepis, however, such an expla- 

 nation cannot be maintained, for here the individuals possessing the 

 smallest chromosome in excess are lacking. Evidently the reason for 

 the absence of certain chromosomal types does not depend entirely 

 upon the relative amount of chromatin present. 



Finally, as was shown above, alterations occur in the organization 

 of the chromosomes themselves in the progeny of triploids. The most 

 interesting variation of that sort is the fragmentation of the satellited 

 chromosome, the latter phenomenon leading to a formation of one 

 very short satellited chromosome (cf. pi. 570 and another longer one, 

 instead of the original single long satellited chromosome. This par- 

 ticular alteration has been found to be the most common one among 

 other instances of reorganization of chromosomes and has been dis- 

 covered in two species (C. capillaris and C. tectorum) . The relative 

 dimensions of both fragments are different in different individuals ; 

 consequently the writer's original suggestion (1926) that there should 

 be a certain "point" in the chromosome where the fragmentation 

 takes place, must be considered incorrect. The cause of this frag- 

 mentation is still ob.scure ; perhaps it is in some way connected with 

 the peculiar behavior of the satellited chromosomes which, as was 

 shown above, are represented in the earlier stages of division by two 

 disconnected parts (the satellite and the body of the chromosome 

 itself) and which are also subject to striking changes in hybrid nuclei 



