398 Intrachromosomal Aberrations 



will be at opposite poles whereas the segment between them will 

 extend from pole to pole as a chromatid bridge (Figs. 105, 106, 

 and 107). At each pole will be one unchanged chromatid, and 

 the fragment will lie on the equator and move toward neither 

 pole. This dicentric bridge chromosome soon will break, and 



Fig. 104. Pairing in salivary gland chromosomes of Drosophila melano- 

 gaster heterozygous for an inversion. (Redrawn from Dobzhansky and 

 Epling in Publication 554 of the Carnegie Institution of Washington [1944].) 



the fragment will be included in one of the two daughter cells. 

 The result will be two cells with one normal and one broken 

 chromatid that will appear more or less normal ; one cell will also 

 have a fragment. The anaphase of the second division will 

 appear more or less normal, although one cell will contain a 

 fragment that will probably not enter into division. The four 

 chromatids can be identified by tracing the chromatids in the 

 prophase drawing at the top of Fig. 105 and allowing for a 

 crossing over at either 1 or 2. 



If there are two crossovers within the inversion involving all 

 four strands, as at 1 and 2 in Fig. 105, first anaphase will consist 

 of a double chromatid bridge and two unpaired fragments. Since 

 each bridge will break during first anaphase, the second anaphase 



