302 CALVIN B. BRIDGES 



conception of crossing over ' chiasmatype' would be misleading, 

 since the term is usually understood as applying to a 'thick- 

 thread, four-strand late growth-stage' condition. The term 

 ' tetraleptotenic' might be used for this type of crossing over to 

 distinguish it from both the 'dileptotenic' and the ' chiasmatype' 

 hypotheses. 



Plough's evidence that the critical stage and crossing over 

 occur after most or probably all of the oogonial divisions have 

 been completed effectually disproves the reduplication hypo- 

 thesis of crossing over as far as any application to Drosophila is 

 concerned, for the number of divisions required by that hypo- 

 thesis is not available. 



Purple has been extensively used in two other important 

 studies on crossing over — that of Sturtevant upon inherited 

 crossing-over variations (in press), and that of Muller (Muller, 

 '16) in his progeny test of a multiple heterozygote in studying 

 crossing over. 



SUIMMARY AND VALUATION 



Because of its excellent characteristics, purple has proved 

 especially useful as a tool in the general work on the second 

 chromosome. In viability, fertility, productivity, and in the 

 many details of 'habit' — ease of handling, activity, time of 

 hatching, length of life, etc. — purple measures well up to the 

 standard of the wild fly. It is separable from the wild type 

 with certainty and rapidity. A difficulty in classification may 

 arise from the occurrence in the same culture of a similar eye 

 color — a 'mimic' or 'pseudopurple' — either by mutation or by 

 introduction. However, the presence of a mimic is generally 

 easily recognized and such difficulties in classification are only 

 temporary. 



The usefulness of purple has not been restricted by 'masking 

 effects': until very recently there has been no other readily 

 workable second chromosome eye color so similar to purple in 

 appearance as to prevent the use of both in the same experiment 

 without confusion between them. Purple is not so dilute that 

 it would interfere with the classification of the eye colors, as 



