352 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 5 



has no corresponding piece in the sister chromosome. It has been 

 shown (fig. 2) that it then conjugates with that part of the parental 

 chromosomes from which it came. 



When a chromosome) has lost one end, it conjugates with its mate 

 only in part (fig. 3a), i. e., where like genes are present. When a 

 chromosome has lost a small region, somewhere along its length, so 

 that it is shorter than the original chromosome, the larger chromo- 

 some shows a loop which is opposite the region of deficiency in the 

 shorter chromosome as shown in figure 3b. Thus like genes, or cor- 

 responding loci, are enabled to come together through the rest of 

 the chromosome. More remarkable still is the case where the middle 

 region of a chromosome has become turned around (inversion). 

 When such a chromosome is brought together with its normal homo- 

 logue, as shown in figure 3c, like regions come together by the in- 

 verted piece reversing itself, so to speak, so that like genes come 

 together as shown to the right in figure 3c. In this same connec- 

 tion the conjugation of the chromosomes in species of Oenothera 

 (fig. 4) furnish beautiful examples of the way in which like series 

 of genes find each other, even when halves of different chromosomes 

 have been interchanged. 



The very recent work of Heitz, Painter, and Bridges has brought 

 to light some astonishing evidence relating to the constitution of 

 the chromosomes m the salivary glands of Drosophila^ figures 5-8; 

 plates 1 and 2. 



The nuclei of the cells of the salivary glands of the old larvae 

 are very large and their contained chromosomes may be 70 to 150 

 times as long as those of the ordinary chromosomes in process of 

 division. Heitz has shown that there are regions of some of the 

 chromosomes of the ganglion cells — more especially of the X and the 

 Y chromosomes — that stain deeply, and other regions faintly (pi. 1, 

 fig. 2), and that these regions correspond to regions of the genetic 

 map that do not and do contain genes. Painter has made the fur- 

 ther important contribution that the series of bands of the salivary 

 chromosomes can be homologized with the genetically known series 

 of genes of the linkage maps (pi. 1, fig. 1, and pi. 2) and that the 

 empty regions of the X and Y do not have the banded structure. He 

 has further shown that when a part of the linkage map is reversed 

 the sequence of the bands is also reversed ; that when pieces are trans- 

 located they can be identified by characteristic bands ; and that when 

 pieces of linked genes are lost there is a corresponding loss of bands. 

 Bridges has carried the analysis further by an intensive study of 

 regions of particular chromosomes and has shown a close agreement 

 between bands and gene location. With improved methods he has 

 identified twice as many bands, thus making a more complete 



