OF MUTANT CHARACTERS. 



251 



have known that the gene for pinkish was in the left end, but this was 

 not the case. 



A test as to whether the pinkish gene would have any visible effect 

 in the absence of eosin showed (table 98) that in a very small per cent- 

 age of the flies homozygous for pinkish there is a very slight dilution. 

 This dilution is, however, so slight that rarely could one be sure that 

 the effect observed is due to dilution rather than to the slight normal 

 fluctuation of the red. 



PLEXUS 



(Text-fig. 80.) 



ORIGIN OF PLEXUS. 



The venation character "plexus" was found by Bridges in a stock 

 culture of the third-chromosome recessive spread wings (August 20, 

 1914, culture 557). Fully 10 per cent of the spread flies showed the 

 plexus venation. 



DESCRIPTION OF PLEXUS. 



The most striking feature of plexus is a rather tangled knot of extra 

 veins near the distal end of the fifth longitudinal vein and the posterior 

 cross-vein, and another 

 such knot near the distal 

 end of the fourth longi- 

 tudinal vein, with an extra 

 vein running near the mar- 

 gins of the wing and con- 

 necting the two (see text- 

 figure 80). Several other 

 small sections of extra vein 

 are often present in various 

 parts of the wing, most of 

 them lying free in the cells, 

 but some being branches 

 of or connected to the 

 regular veins . These veins 

 are all sharp and clear, 

 without indefiniteness and 

 discoloration such as char- 

 acterize the extra veins of 

 balloon. There is a very 

 characteristic bend forward 

 in the fourth longitudinal vein before it reaches the marginal vein. 



In less extreme individuals the connecting vein may be quite absent 

 and the knots much reduced. The branch from the posterior cross- 

 vein running parallel to the fifth longitudinal vein and also the bend 

 in the fourth vein are the most persistent of all the characters. 



TEXT-FIGURE 80. Plexus venation. 



