132 CARNEGIE INSTITUTION OF WASHINGTON. 



in a pedigree heterozj'-gous for spines and node number as well as for color. 

 Although detailed field records have not yet been taken, the segregation of 

 these other factors appears to conform to the expectation made out for the 

 purple stem-color. Triple recessivcs have not appeared, but the theoretical 

 chances of their occurrence are only 1 in 46,456 individuals in this generation. 

 They should be easily obtained another year. 



"So far as has been determined, the Datura mutants of the 'Globe' type 

 seem to be due to the presence of an extra chromosome giving 25 instead of 24 

 chromosomes in the somatic cells. The gametes therefore would have 12 and 

 13 chromosomes. There appears to be some evidence that the extra chromo- 

 some is a specific one in each case. Apparently we have been able to identify 

 in Poinsettia the mutant which has as an extra chromosome — the one carry- 

 ing the determiners for purple and green stem-color. Five Poinsettia plants, 

 heterozygous for purple and green, have, when selfed, produced pedigrees 

 of which all the Poinsettia mutants are purple and the normals show purple 

 and white in a ratio closely approximating 8:1. If these parent Poinsettia 

 plants be considered to have the formula PPp, their female gametes should 

 be P+Pp+P+Pp+P+PP or 2P+2Pp+p+PP. The male gametes repre- 

 sented by the pollen-grains should be the same, but, since the mutant char- 

 acter generally fails to be carried by the pollen, the effective male gametes 

 may be considered to be 2P+p. Selfing Poinsettia plants of the formula 

 PPp should therefore give all the Poinsettia offspring purple and 8 purples 

 to 1 green among the normals, a result that was actually obtained. The 

 matter is being tested further with this particular mutant and an effort is 

 being made to discover which of the remaining mutants have as their extra 

 chromosomes those carrying factors for known Mendelian characters. It 

 seems inadvisable to report further than in a very tentative way on the Globe 

 type of mutations in Datura, since the present stage of the work would not 

 warrant any definite conclusions. It is obvious, however, that if the facts 

 turn out to be what the present findings indicate, they may furnish the clue 

 to much which has been unintelligible in this and certain other species." 



RudhecJda. — The breeding work on Rudbeckia has been discontinued 

 by Dr. Blakeslee on account of the difficulties of technique involved in 

 crossing them, and theh decrease in vigor when inbred. The work on 

 the inheritance of the tw^o yellow cones in this species and their iden- 

 tification by chemical means has been -vvTitten up and is now in press. 



HEREDITY OF SUSCEPTIBILITY TO CANCER. 



The importance to mankind of a knowledge of the hereditary factors 

 in cancer can hardly be overestimated, and the theoretical interest of 

 an analysis of the behavior of these factors is no less great. Dr. Little 

 reports on his results in this field as follows : 



" Genetical factors involved in susceptibility. — In 1916, Little, in collaboration 

 with Tyzzer, reported on the inheritance of susceptibihty to a transplantable 

 carcinoma (J. w. A.) of the Japanese waltzing mouse. This tumor grew in 

 100 per cent of the Japanese waltzing mice inoculated, and in per cent of 

 the common non-waltzing mice. When these two races were crossed, the Fj 

 generation hybrid showed 61 out of 62 mice to be susceptible. The F2 genera- 

 tion gave a very interesting result — only 3 out of 183 mice grew the tumor. 

 At that time the results were explained on the basis of multiple Mendelizing 



