ON THE ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 425 



second case a chain of 8 or 9 cells of Ghaetoceras densum, picked out 

 from a Petri dish culture, has given a particularly good growth. 



The method of dilution and subdivision has been more successful 

 and persistent cultures 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 cc. 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 sttiL The temperature should be kept as constant as possible and 

 the dishes exposed to light of moderate intensity, 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 early 

 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 a single species would not be obtained. The 

 above method is similar to one described by Miquel, excepting that he 

 placed gelatinous silica at the bottom of the vessel. Some very 

 successful persistent cultures were obtained from the following experi- 

 ment, which will serve to illustrate the method : — A sample of plankton, 

 from a very fine-mesh bolting-silk tow-net, was diluted down with 

 sterile sea-water, until a single drop examined under a two-thirds-inch 

 objective contained on an average ten organisms, chiefly diatoms of 

 various species. Petri dishes (4 in.), containing 60 cc. each of Miquel 

 sea-water, were then inoculated with various numbers of drops of the 

 diluted plankton. The two dishes, to which two and three drops 

 respectively were added, gave the best results ; and from these per- 

 sistent cultures of several species of diatoms were obtained Hence 

 we may conclude that the most advantageous number of single cells or 

 short chains of cells to be added to a 4 in. Petri dish, containing 60 cc. 

 culture medium, is about 20 to 30. 



We have succeeded in obtaining the following species of Plankton 

 diatoms in persistent cultures : — 



Asterionella japonica, Cleve. 

 Biddulplda mohiliensis (Bail.), Grun. 

 BiddvJpliia rcfjia (M. Schultze).* 



* See pp. 461. 



