ENVIRONMENTAL EFFECTS 



be seen. Subsequent complete terminalization 

 and separation of dyads in anaphase I leaves 

 fragments and there are no bridges. A broken 

 dyad produces a bridge in division II in either 

 spindle with equal frequency. If a single chro- 

 matid is broken the fragment remains attached to 

 its uninjured partner and appears in division 

 II. An egg with a single deletion will fail to 

 hatch if unfertilized, and, even if fertilized, 

 when the deletion is large.* If two chromatids 

 in a "lug" (distal to chiasma) were broken and 

 the chromatids fused laterally, an occasional 

 bridge in division I might be expected. Tension 

 is much less in this region so that restitution 

 usually occurs when a hit causes a break in 

 this short length of chromosome. As explained 

 above, an egg with one double fragment has one 

 chance in two of hatching, an egg with two such 

 fragments, one chance in four (if the possibil- 

 ity of two breaks in one dyad be left out of 

 consideration) . 



In Habrobracon, bridges in division II fol- 

 lowing treatment in late- metaphase I cannot be 

 explained by chromosome splitting following 

 breakage in view of the structure of the meta- 

 phase tetrad. It therefore seems probable that 

 a single hit has broken two chromatids in late 

 metaphase I. It is known that many deficiencies 

 are viable in the heterozygous condition, le- 

 thality being dependent on gene content of the 

 segment lost. Since diploid larvae are most re- 

 sistant to irradiation than haploid, recessive 

 deficiencies must be induced in somatic cells. 

 It is surprising, in view of these facts, that 

 all the lethal effects of irradiation appear to 

 be dominant. If breaks proximal to the chias- 



135 



