254 



THE SECOND-CHROMOSOME GROUP 



of the interval, especially since the amount of double crossing-over 

 and of coincidence in this region are known only, by inference from 

 other experiments, there being no intermediate locus by means of 

 which direct calculation could be made. Arc could be used for this 

 purpose, but is unsuitable because of a probable confusion in classi- 

 fication with curved. Plexus, on the other hand, being a venation 

 character only, can readily be used with curved, and its position is 

 more favorable than arc, since it more nearly divides the gap. 



Preparations were made to make an extensive experiment which 

 should give data on the plexus speck distance as well as on several 

 others throughout the length of the chromosome. But as yet this 

 has not proceeded further than the synthesis of the multiple recessive 

 needed (d b Pr c p^ Sp), and of the two parent stocks necessary for an 

 "alternated" experiment (S' b c Sp and d Pr p^). 



A summarj'- of the crossing-over data imvolving plexus is given in 

 table 101. The locus of plexus is about 8.9 units to the left of speck, 

 or at 96.2. 



VALUATION OF PLEXUS. 



Sturtevant has reported trouble in the classification of plexus in 

 certain crosses, and it is not certain that all the plexus individuals can 

 be separated from wild-type where the variation is great. In none of 

 the experiments here reported was difficulty encountered. 



LIMITED. 



(Fig. 81.) 



In carrying out the black arc morula back-crosses (culture 513, 

 September 13, 1914), Bridges noticed that there was present a charac- 

 ter somewhat similar to abnormal abdomen, 

 except that its main effect was evident on the 

 ventral surface of the abdomen and to a slight 

 extent on the side. The chitinous ventral plates 

 on the abdomen, instead of being full size with 

 rounded edges and many regularly arranged 

 small hairs (as in fig. 81), were reduced often to 

 half the size by an irregular erosion of the edges. 



The color also was etched. The hairs were very 

 few in number, and those irregularly arranged 

 and directed. The dorsal plates where they 

 bent around to the ventral side were affected in 

 the same manner at their ends, though not so Text-figure si.— Limited 



strikinfflv bands to the abdomen. 



This character was almost entirely limited to the morula flies and 

 was distributed in such a way that its locus was certainly second- 

 chromosome and probably to the right of morula, though the counts 

 were not made carefully enough to be sure of this. 



