OF MUTANT CHARACTERS. 197 



it seems probable that crossing-over occurs at a thin-thread stage, or 

 at least that the characteristic transjunction is accomplished at a 

 leptotene stage, such as occurs only in the early growth-period, liut 

 such a conception does not exclude the possibility that the crossing- 

 over occurs at a four-strand stage, as is indicated by still other lines 

 of evidence. To call such an early-growth-stage, thin-thread, four- 

 strand hypothesis of crossing-over " chiasmatj^De " would be mislead- 

 ing, since the term is usually understood as applying to a late-growth- 

 stage, thick-thread, four-strand condition. The term "tetralepto- 

 tenic" might be used for this type of crossing-over to distinguish it 

 from both the dileptolenic and the chiasmatype hypotheses. 



Plough's e\'idence that the critical stage and crossing-over occur 

 after most or probably all of the oogonial divisions have been completed 

 effectually disproves the reduplication hypothesis of crossing-over as 

 far as any application to Drosophila is concerned, for the number of 

 divisions required by that hypothesis is not available. 



Purple has been extensively used in two other important studies on 

 crossing-over — that of Sturtevant upon inherited crossing-over vari- 

 ations (Part III of this volume), and that of IMuller in his progeny 

 test of a multiple heterozygote in studying crossing-over. (]Muller, 1916). 



SUMMARY AND VALUATION OF PURPLE. 



Of the 200 or more mutations in Drosophila, certain ones have proved 

 especially useful as working tools because of excellent characteristics 

 or favorable location in the chromosome. Certain others have an 

 even higher interest because of their intimate connection with the 

 development of principles or subjects that have now come to be the 

 groundwork of every Drosophila experiment. 



As a working tool the second-chromosome recessive eye-color purple 

 has deserved its very extensive usage. In viability, fertility, produc- 

 tivity, and in the 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. In separability from the wild-type, purple is satisfac- 

 tory both in certainty and in speed. The only faiUng in certainty 

 is that arising from the occurrence in the same culture of a similar eye- 

 color — a "mimic" or "pseudo-purple" — 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 ease and rapidity of separation fail to be satisfactory with purples 

 older than about 3 days, though rarely is there any need for separa- 

 tions so delayed. 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 



