Tlie structure of the internal Genitalia of some male Diptera. 503 



method. A still greater drawback attended vom Kath's mixture and 

 that was the difficulty of washing out the picric acid. Of course one 

 can try washing out and staining the sections on the objectglasses, 

 but for a research as this one, in the course of which I had to make 

 so many sections, that about 300 objectglasses of a dimension about 

 4x2V2 inches were necessary, washing and staining takes too much 

 time in practice. Still I must point out that in preparing the sections 

 by passing them successively through xylolum, alcohol of different 

 percentage, colour-liquid and back to xylolum, the chitin will t'^ar off. 

 Even the use of a mixture of glycerine and the white of an egg is not 

 sufficient to obviate this defect, while here also the sections often 

 folded and the albumen became fixed by the alcohol, got impregnated 

 with the colour and was not easily washed out. The same faults as 

 with VOM Rath's mixture, accompany the fixation-method of Janet 

 too, with this difference, that the Solution penetrates more easily in 

 the objects. 



Then trying Carnoy's mixture (1 part acetic acid, 3 parts absolute 

 alcohol) I found that several difficulties were to a great extent remo- 

 ved. The fixation-Iiquid enters much more easily and pushes away 

 the air, causing many objects to sink. During the microscopical 

 research it seemed to me however, that the tissues were swollen too 

 much, probably as a consequence of a too great percentage of acetic acid. 



After that, I used to fix with a mixture of 1 part acetic acid with 

 9 parts absolute alcohol and dropped into this Solution the abdomina, 

 which I cut off from the living Diptera, and then I put the liquid 

 under the glassbell of an airpump, while I pumped away the air very 

 carefully, and toke care, that the thinning of the air was not too great. 

 At first I feared, that this procedure would prove injurious to the 

 Organs, but as I set to work with the utmost caution, it became evident 

 to me by and by that this fear was unfounded. When I was not able 

 to keep alive, for some considerable time, the insects I had caught 

 and to fix them under the glass-bell of the airpump, then I made a 

 small sagittal cut from the dorsal region into the abdomen and after 

 that I put the objects in the fixation-Iiquid. 



The objects were coloured in toto in a waterish Solution of haema- 

 lum or ironcarmalum. I must point out, however, one little difficulty, 

 that I experienced with these objects which were fixed under the 

 glassbell of the airpump, and that is, that I sometimes found the caudal 

 part of the abdomen to be less intensively coloured than the cranial 

 one, from which the colour had chiefly penetrated, moreover the im- 



33* 



