442 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 30 



is our old friend the white chromosome. The chromosome 5 • 6 is 

 " Buckling " (Bk) and 9 • 10 is " Echinus " (Ec). The chromosomes 

 5 • G and 9 • 10 are in pairs but there is only one normal 1 ■ 2 chromo- 

 some present. However, the "Pinched" (Ph) chromosome (5-2) 

 furnishes the 2 half and the "Hedge" (Hg) chromosome (1-9) 

 furnishes the 1 half of the missing 1 • 2 chromosome. The portions 

 1 and 2 in these two chromosomes are said to compensate to give the 

 equivalent of a normal 1 • 2 chromosome. There is left over as extra 



chromosomal material, therefore, the 5 

 half and the 9 half chromosomes. 

 The 5 • 2 and 1 • 9 chromosomes are ter- 

 tiary chromosomes which, as extras in the 

 types Ph and Hg, are thrown by the 

 compensating type Nb. (PL 13, fig. 1.) 

 The related chromosomes involved in 

 Nb may be given by the following for- 

 mula : 



^ 



^ 



m 



Figure 5. — Models of the chro- 

 mosomes involved in the com- 

 pensating type " Nubbin." The 

 chromosomes 5 • 6 and 9 • 10 

 are in pairs but only a single 

 1 • 2 chromosome is present. 

 However, the tertiary 5 • 2 

 chromosome furnishes the 2 

 half and the tertiary 1 • 9 

 chromosome furnishes the 1 

 half of the missing 1 • 2 chro- 

 mosome. The 5 and 9 

 parts are left over as extra 

 chromosomal material to bring 

 about the peculiarities of Nb 



9 • 10—10 • 9—9 -l—l- 2—2 • 5—5 • 6—6 • 5. 

 The dashes indicate the connections of 

 the chromosomes at the reduction divi- 

 sions. The chromosomes of Nb, as of 

 other compensating types with two ter- 

 tiary chromosomes, form a chain of seven 

 attached members. In this as in the fol- 

 lowing cases the parts which compensate 

 are shown in italics and the parts which 

 remain as extra chromosomal material 

 are shown in boldface type. 



MD is another compensating type 

 with the same chromosome (1-2) compensated. Its formula is: 



9-10— 10-9— 9-./— 1-2— ;g-17—17-18— 18-17. 



MD shows some resemblance to Nb because of the 9 half chromosome 

 which is in excess in both types. Its appearance, however, is affected 

 more strongly by the excess 17 half and in consequence its resem- 

 blance to Nb is not great. 



The formula of another compensating type which is not yet named 

 is: 



13 • 14— 14 • 13—13 -11— II ■ 12—12 • 17—17 • 18—18 -17. 



In this case the 11-12 chromosome is compensated and the extra 

 chromosomal material is 13 and 17. This type shows some resem- 

 blance to MD on account of the 17 half chromosome which is extra 

 in both types. Because the remaining extra chromosomal material 

 is not the same in the two types the two forms are not alike in ap- 

 pearance. 



