462 ON THE ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 



seems to reach a limit, where further decrease does not take place. 

 In chains of Thalassiosira, several hmidred cells in length, no difference 

 in size between individuals could be made out. Auxospores are 

 commonly formed with every species, but only in the cultures of 

 Coscinodiscus and Thalassiosira have megafrustules been found, and in 

 these they are very exceptional. These megafrustules seem to divide 

 once or twice and then die or form new auxospores. What exactly is 

 tlie fate of these auxospores, which are often exceedingly numerous, 

 we have not been able to make out. It seems that cultural conditions 

 are not favourable to this mode of reproduction, and that the auxo- 

 spores do not further the multiplication of the diatom at all. If this 

 were not the case, stages of the formation of auxospores into frustules 

 must have been seen in some at least of the very numerous samples 

 examined. As it is, what has been said to take place is, that the cell 

 contents expand and force apart the valves of the diatom and emerge 

 as a spherical body about three or four times the diameter of the parent 

 cell. The chromatopliores and diatomin then collect to one side, form- 

 ing a compact cap against the cell wall. Beyond this point no stages 

 have been found, except in the case of the few cultures where mega- 

 frustules were formed. In these the chromatophores, etc., gradually 

 formed into the shape of the diatom {Coscinodiscus), the silicious coat 

 with plain eccentric markings was easily seen inside the spore, and 

 lastly, t]i9 cell wall of the spore burst, leaving the megafrus'tule free. 

 The megafrustule was measured and found to have a diameter tln-ee 

 times that of the parent cell. 



In the case of the diatom we have very largely used for feeding 

 larvie, etc., namely NitzscMa clostfrinm, forma 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 diminu- 

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

 of re-establishment of size has been seen, although these cultures have 

 been under constant observation for over two years. This seems to 

 prove that the theory of gradual decrease in size with successive 

 generations cannot be generally applied. 



The following experiment on the rate of multiplication of Thalas- 

 siosira 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 tlie results are given 

 in tlie following; table : — 



