286 



THE AMERICAN NATURALIST 



[Vol. LVI 



When the entire amount of water has passed through the filter- 

 paper, the latter is removed, spread out, and immersed in a 

 bath of water, in a watch crystal. The water should just cover 

 the filter-paper. 



The device shown in Fig. 2 is now brought into play. This 

 consists of a glass pipette, flattened and spread at its tip, and 

 serves admirably for gently scraping and sucking the surface of 

 the filter-paper, as it lies in the watch crystal. This withdraws 

 into the pipette the organisms which have been filtered out. 

 These can now be transferred to a glass slip and examined under 

 the microscope, or injected into culture media as inoculations. 



The writer has found that, with practice, the possibilities of 

 the micro-filter may be extended to aid, in many ways, in the 

 study of the protozoa. 



Leon A. Hausman 



COMPLETE LINKAGE IN DROSOPHILA MELANO- 

 G ASTERN 



In 1917 a mating appeared in the cultures of the authors, 

 the flies from which showed no crossing over in the region 

 scute to forked of the sex chromosome, although the factors 



points. This culture appeared spontaneously; selection played 

 no part in it. The stock from this culture has now passed 

 through not less than 80 generations and numbers over 3,000 

 matings. During this time no crossing over has appeared 

 within the known length of the sex chromosome. 



In experiments including the second chromosome points, 

 black and purple, it has been shown that no crossing over takes 

 place between these points when complete linkage exists for 

 the first chromosome. Likewise the third chromosome points, 

 dicheate and hairless, have shown complete linkage when the 

 points scute to forked in the first chromosome, and the points 

 black to purple in the second chromosome show the same 

 phenomena. 



The disturbing cause is genetic, behaving as a recessive. Its 

 1 Papers from the Biological Laboratory, Maine Agricultural Experiment 

 Station, No. 142. 



