366 E. .1. ALLEN AND E. W. NELSON. 



obtained persistent cultures, containing a single species of 

 diatom, by both of the methods recommended by Miquel. We, 

 however, have i-arely succeeded ])y picking out single diatoms 

 or chains of diatoms, for although we have passed the selected 

 diatom through several changes of sterilised sea-water, the 

 resulting cultures, even when the diatoms have multiplied to 

 some extent, have generally shown evidence of contamination 

 by harmful organisms, and have soon died down. Only in 

 one of the earliest experiments, and in one more recent, has 

 complete success resulted. In the first case a small chain of 

 six or eight frustules of Skeletonema costatum, picked 

 out in April, 1905, gave rise to a culture which still persists 

 (November, 1909). Subcultures can still be obtained even 

 from the original flask inoculated in April, 1905. In the 

 second case a chain of eight or nine cells of Chaetoceras 

 densum, picked out from a Petri dish culture, has given a 

 particularly good growth. 



The method of dilution and subdivision has been more suc- 

 cessful, and persistent cultui'es of a number of species have 

 been obtained in this way. 



A more ready method of obtaining the cultures is, we have 

 found, to add one or two drops of plankton to, say, 250 c.c. 

 of a suitable sterile culture medium, and to pour this into 

 shallow glass dishes (Petri dishes). The dishes should be 

 placed in a position as free as possible from vibration, and 

 where they can be easily examined with a lens in situ. The 

 temperature should be kept as constant as possible and the 

 dishes exposed to light of moderate intensit}', direct sunlight 

 being avoided. In the course of a few days, colonies of 

 diatoms of different species will be seen at different spots on 

 the bottom of the Petri dishes. These can be picked out 

 with a fine pipette and transferred to flasks containing fresh 

 culture medium. The colonies should be picked out from the 

 Petri dishes at as earl}' a stage as possible, because if left too 

 long some one organism, a diatom or a flagellate, may have 

 multiplied so rapidly that the whole of the water in the dish 

 becomes infected with it. In this case persistent cultures of 



