1180 BIOLOGICAL EFFECTS OF RADIATION 



The discovery of crossing over in Drosophila males induced by X-rays 

 (Patterson and Suche, 110; Friesen, 42) may conceivably have a bearing 

 on the problem of the origin of induced translocations. Since, however, 

 in this case breakages and reattachments take place in chromosomes 

 homologous to each other, and apparently always at the same level, a 

 comparison of the crossing over in the male with translocation is very 

 hazardous. 



PERMANENCE OF SPINDLE FIBERS 



The location of the attachments of the spindle fibers in the chromo- 

 somes of Drosophila melanog aster was determined before the discovery 

 of the cytologically visible translocations and other chromosomal rear- 

 rangements. At anaphases and telophases the spindle attachments lie 

 closest to the poles of the spindle, and the free ends of the chromosomes 

 are directed away from the poles. Recently Kaufmann (56) has made 

 a more exact cytological determination of the location of the spindle 

 attachments by observing the achromatic breaks in the prophase chromo- 

 somes. In the large V-shaped autosomes (the second and the third 

 chromosomes) the spindle attachments lie at the apices of the V's, in the 

 J-shaped F-chromosome at the junction of the two limbs, and in the X- 

 and the fourth chromosomes the attachments are subterminal. 



The localization of the spindle attachments was determined geneti- 

 cally by Bridges and Morgan (19) on the basis of distribution of double 

 cross-overs, by L. V. Morgan (70), Anderson (4), and Redfield (111, 112) 

 from data on equational exceptions, by Stern (126, 127) from studies on 

 the interactions of bobbed allelomorphs located in the X- and F-chrorao- 

 somes, by Sturtevant (131, 133) and Dobzhansky (24, 26, 28, 29) from 

 disturbances of crossing over produced by inversions and translocations, 

 and by Bolen (11) by observing duplications for sections of the fourth 

 chromosome. The location of the spindle attachments in terms of the 

 genetic maps of chromosomes is shown in Figs. 3, 5, and 6. The results 

 of the cytological studies on translocations completely corroborated the 

 earlier determinations of the location of the spindle attachments based on 

 purely genetic data. In every case in which breakages of chromosomes 

 were found genetically to lie close to the genetic loci of the spindle 

 attachments, cytological studies showed the chromosomes to be broken 

 near the attachment constrictions (MuUer and Painter 80, 81; Dob- 

 zhansky 24, 26, 27, 29, 31, 32). 



The discovery of translocations and other chromosome rearrange- 

 ments which produced visible alterations of chromosomes furnished a 

 method for an experimental attack on the problem of permanence of 

 spindle attachments. Some inverted sections were found to involve 

 genes lying on both sides of the attachment constrictions. In such 

 inversions in the second chromosomes of Drosophila the loci of the spindle 



