ARTIFICIAL GULTUE.B OF MARINE PLANKTON OROANLSMS. 415 



minutissima, a great number of cultures have been made, 

 all originating from the single drop from which the first 

 persistent culture was obtained. The total amount of growth 

 in all the various cultures has been enormous, and the number 

 of generations must be quite inconceivable. No diminution in 

 size has, however, been appreciable, and no sign of any method 

 of re-establishment of size has been seen, although these cul- 

 tures have ])een under constant observation for over two years. 

 This seems to prove that the theory of gradual decrease in 

 size with successive genei"ations cannot be generally applied. 

 The following experiment on the rate of multiplication of 

 Thalassiosira in normal Miquel sea-water was carried out. 

 A single drop from a fresh and vigorous culture was kept 

 under a microscope as a hanging-drop preparation in a moist 

 chamber. The number of diatoms in this drop was counted 

 from time to time and the results are given in the following- 

 table : 



The curve obtained b}' plotting the number of diatoms 

 against the number of days approximates the curve of an 

 ordinary geometric progression, where the ratio is 2 and 

 the periods ai'e equal to sixteen days. To show this the 

 figures read o^ from the curve at the same intervals as 

 the diatoms are appended in the table. From this it will be 

 seen that, after a start had been made and before exhaustion 

 set in, the numbers obtained agree fairly closely with the 

 assumption that every diatom divided once in a period of 

 sixteen days. Probably in normal cultural conditions the 

 rate of multiplication greatly exceeds this figure on account 

 of better lighting, etc. (cf. Miquel 12). 



