Net. 53 



those of bent, described above. It is thought that these may be homozygous, but 

 the few found thus far have failed to breed. 



Net exhibits as many pecularities in behavior as it does in appearance. It ia 

 a dominant and is lethal, or nearly so, in a homozygous condition. No fertile homo- 

 zygous flies have been obtained. The viability of net files is very poor; their de- 

 velopmental period is longer than that of most races; they begin hatching from two 

 to four daj's later than their normal sibs, and they usually fall far behind expectation 

 in number. 



Origin. — (L 462.) Net was first recognized among the offspring of a pair mating 

 in which both parents were heterozygous for telescoped and hump and one parent 

 was heterozygous for pinched. Only one fly was observed to be net, but it is possible 

 that other net flies would have appeared if the culture had been a good one. 



The peculiarities of net suggest that it is a "deficiency" instead of 

 an ordinary gene mutation, and that it may be analogous to Bridges' 

 (1921) "diminished" in D. melanog aster, with which it agrees fairly 

 closely in appearance and behavior. Diminished is due to the ab- 

 sence of one of the two small 7n-chromosomes. In our material 

 (of net), however, cytological examination has shown that all of 

 the chromosomes are present. But crosses with bent support 

 the conclusion that net is a deficiency and that it is probably due 

 to the absence or inactivation of a part of a chromosome containing 

 genes homologous to those in the m-chromosomes ("fourth chromo- 

 some") of melanagoster. 



When net and bent are crossed the Fi net flies are all bent. When 

 Fi net bent males are back-crossed to bent females the offspring 

 are of only two classes, net bent and bent. This indicates that net 

 and bent are in the same Unkage group (chromosome) and also 

 that net is a deficiency for bent. Corroborative evidence is furnished 

 by the fact that both net and bent show independent segregation 

 with characters in the other five linkage groups. Fi net bent females 

 have not been back-crossed, because all that have been tested 

 (about a dozen) have been sterile. 



