ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 371 



To each 1000 c.c. of sea-water^ add 2 c.c. solution A and 

 1 c.c. solution B and sterilise by heating to 70° C. When cool, 

 decant off the clear liquid from the precipitate, which will 

 have formed when solution B is added to the sea-water. 



As a rule our cultures were made in 60 c.c. of this medium 

 contained in short-necked, wide-mouthed flasks of 125 c.c. 

 capacity, so that the proportion of air-surface to volume of 

 liquid was large. 



The medium was found to give constantly satisfactory 

 results. On inoculation from a persistent culture of such 

 diatoms as Thalassiosira, Skeletonema, Chsetoceras, 

 etc., a growth visible to the eye is obtained in about ten days, 

 and then multiplication takes place very rapidly. In from 

 three weeks' to a month's time a very considerable growth will 

 be seen making a brown, flocculent mass at the bottom and 

 back of the vessel containing the culture. 



In from two to four months the culture begins to show 

 signs of exhaustion and the frustules lose colour, but they do 

 not, as in the case of sterilised outside and tank-water, com- 

 pletely die off. A great number certainly do die, but some 

 remain in a resting condition, and often, after a period of six 

 months or so, these begin to multiply again and the culture 

 regains its former vigour. This is probably due to the food- 

 stuffs contained in the dead frustules going into solution again, 

 possibly by means of bacterial action. This periodicity in 

 cultures is interesting in that it resembles what takes place in 

 the ocean. Cultures in this medium will persist indefinitely, 

 so far as our experience goes. The oldest culture in our 

 possession is one of Skeletonema costatum made at the 

 very commencement of this work, dated April, 1905. Although 

 the frustules in this culture ai-e quite unrecognisable as any 

 diatom now, on making a subculture in fresh Miquel a normal 

 and healthy growth can always be obtained. 



In old cultures the diatoms are nearly always found to be 

 very much deformed, and often appear to be only a mass of 



1 " Miqiiel water " seems to succeed equally well, whether it is made 

 by adding Miquel's solutions to "outside water" or to "tank -water." 



