ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 247 



B. Technique.* 

 (1) Collecting: Objects, including Culture Processes. 



Use of the Centrifuge in Pond-life Work. — The application of the 

 centrifuge to the collection of minute organisms in water was apparently- 

 first suggested, says D. J. Scourfield,t by Cori in 1895, and very soon 

 after, Dolley, Kofoid and Field were also experimenting in a similar 

 direction. During the next ten years or more, however, the method 

 was not at all widely adopted, probably because it was looked upon 

 merely as an alternative to the more usual methods of concentration by 

 means of fine gauze nets, etc. It was not until Lohmann in 1908 pub- 

 lished his paper on the determination of the absolute quantity of 

 plankton in sea-water t;hat it was realized that the centrifuge was indis- 

 pensable so far as the minutest forms of plankton were concerned. 

 Immediately after the publication of Lohmann's paper, "Woltereck and 

 Ruttner took up the matter at the Lunz Fresh-water Biological Station, 

 and have recorded some preliminary but extremely interesting results 

 in the Internationale Revue der gesamten Hydrobiologie und Hydro- 

 graphie. 



The samples of water to be tested with the centrifuge should be 

 taken directly from the pond or other piece of water into the collecting- 

 bottle without the intervention of any net or other filtering appliance. 

 Quite small quantities are sufficient for obtaining a fairly accurate idea 

 not only of the various kinds of smaller organisms present but also of 

 their relative numbers. With water from small lakes and ponds, tubes 

 holding only 1| c.cm. usually yield sufficient deposit for qualitative 

 investigation, though not perhaps for accurate quantitative work. 



As some organisms seem to remain suspended in the water for an almost 

 indefinite time if the centrifugal force does not reach a certain amount, 

 it is necessary, if these are to be concentrated, to run the centrifuge at 

 a high speed. To obtain this with manual power the author has had 

 the " haeraatocrit " head supplied with all two-speed centrifuges, some- 

 what altered so as to take elongated vase-shaped tubes holding no more 

 than about 1^ c.cm. With such a head, speeds up to 10,000 revolutions 

 per minute can be obtained, but the highest speed obtainable without 

 too great an effort is about 7,000 revolutions per minute. At this speed 

 a run of from one to two minutes seems to be sufficient for the purpose. 

 It is useful sometimes first of all to centrifuge samples of water in the 

 larger tubes usually supplied (holding about 15 c.cm.) at a comparatively 

 low speed, and then to pipette off some of the clearer water into the 

 smaller tubes for more rapid rotation with the high-speed gearing. This 

 has the great advantage of separating to a considerable extent the 

 larger from the smaller of these tiny organisms, and so rendering the 

 examination of the latter more easy. ' It would probably be a good plan 



* This division contains (1) Collecting Objects, including Culture Pro- 

 cesses; (2) Preparing Objects ; (3) Cutting, including Embedding and Microtomes; 

 (4) Staining and Injecting ; (5) Mounting, including slides, preservative fluids, etc. ; 

 (6) Miscellaneous. 



t Journ. Quekett Micr. Club,ser. 2, xi. (1911) pp. 243-50 (1 fig.). 



S 2 



