ON THE ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 461 



present, although this is not always the case. The odd asymmetrical 

 apiculus can nearly always be seen. The eccentric markings have only 

 been observed in a few isolated cases, and are then usually very indis- 

 tinct. In one culture these markings on the valves were very distinct, 

 and were also easily seen on the megafrustules (cf. below) which 

 developed in it, but in none of the several generations of cultures 

 started from this one have we been able to find any traces of marking 

 at all. The genus seems to be in considerable confusion, and it is prob- 

 able that the conflicting descriptions given by different observers are 

 due to variations in what is really one species. 



Persistent cultures of Coscinodiscus excenfricus, Ehr., have also been 

 obtained, and it is interesting to note that this diatom sometimes forms 

 chains, but they are rather exceptional. These chains are never as long 

 as those commonly found with Tlialassiosira, two or four cells only being 

 the rule. The filament joining the valves is also finer and more easily 

 broken. The two species are quite distinct, and cultures of them can 

 be discriminated by a practised eye. 



Two species of the genus Biddulphia are commonly met with in 

 our cultures, namely Biddulphia mohilicnsis (Bail.), Grun., and Bid- 

 didphia regia, M. Schultze. These two forms are generally regarded 

 as one species, but Ostenfeld (54) has recently shown that they are 

 really distinct. We have obtained persistent cultures of both forms 

 from several different samples of plankton, and the two species are 

 easily recognizable, never merging into one another. When Petri 

 dishes, inoculated from plankton (see p. 425), contain both species, 

 tlie colonies can be easily distinguished with a small hand 

 lens. 



The most generally accepted theory of the reproduction of the 

 diatomaceffi is briefly that the cells divide by simple fission, but on 

 account of the rigid character of the cell walls each division necessitates 

 a decrease in size of the new valve, since this must always be formed 

 inside the old valve. So the frustules gradually get smaller and 

 smaller as multiplication proceeds, thus necessitating some process by 

 which the original size can be re-established. This takes place by the 

 formation of what are known as auxospores, which ultimately form 

 megafrustules, and these in turn multiply by division until the 

 minimum limit of size has again been reached. There are also several 

 special processes of reproduction, but no occurrence of any of these has 

 been noted in our work (cf. Miquel, 14). 



The diatoms in our cultures multiply by simple fission, and although 

 there is, in nearly every case, a considerable diminution in size when 

 compared with specimens from the plankton, this diminution soon 



