444 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 30 



Secondary (2n + l) types occur spontaneously, but much less fre- 

 quently than the primaries. There is no known method oi' obtain- 

 ing them in large numbers. 



Tertiary (2n+l) types have been obtained by a number of dif- 

 ferent methods, (a) A few have occurred spontaneously in our 

 cultures, (b) They have been secured from cryptic chromosomal 

 races in nature, such as the "B" races. (Fig. 4.) By crossing 

 the " B " race, for example, onto a (2n + l) type in which the extra 

 chromosome is related to the modified chromosomes of the " B " race, 

 a (2n + l) tertiary may be secured in the next 

 generation. By this method we have secured 

 the tertiaries Wy (pi. 10, fig. 2) and DS. (pi. 

 10, fig. 1). (c) Since it is known that other 

 species differ from our standard line 1 in the 

 arrangement of the parts of their chromosomes 

 more than does the " B " race, these other spe- 

 cies probably afford another rich source of ter- 

 tiary forms. Some success has already been 

 had in seeking tertiary forms from this source. 

 (d) The most prolific method of obtaining ter- 

 tiary chromosomes, liowever, is treatment with 

 radium or X rays. Under this stimulus, non- 

 homologous chromosomes are broken and their 

 parts interchanged to produce tertiary chromo- 

 somes such as were described for the " B " race. 

 (Fig. 4.) In hybrids between "B" race and 

 our standard line 1 the chromosomes at the 

 reduction division are seen to be connected to- 

 gether in a circle of four. (Fig. 6.) This con- 

 figuration results because like ends of chromo- 

 somes are attached together at the reduction 

 division. In a similar manner circles of at- 

 tached chromosomes result when radiation has induced segmental 

 interchange between two nonhomologous chromosomes. Thus fol- 

 lowing radiation treatment, the chromosomes 1 • 2 and 13 • 14 inter- 

 changed segments to form the tertiary chromosomes 1 • 14 and 2 • 13. 

 The following circle resulted : 



Figure 6. — Models of tbc 

 circle of four attached 

 chromosomes in hy- 

 brids between the 

 standard line 1 and a 

 " B " race. For chro- 

 mosomal diagrams of 

 the parental races see 

 Figures 3 and 4 



1-2 



2 13 



114—1413 



The tertiary chromosomes are represented in bold-face type. An- 

 other configuration following radiation was a circle of four as 

 follows : 



13-14— 14-24 



13-23—23-24 



