414 E. J. ALLEN AND E. W. NELSON. 



There are also several special processes of reproduction, hut 

 no occurrence of any of these has been noted in our work 

 (of. Miquel [14]). 



The diatoms in our cultures multiply by simple fission, and 

 although there is, in nearly every case, a considerable diminu- 

 tion in size when compared with specimens from the plankton, 

 this diminution soon seems to reach a limit, where further 

 decrease does not take phice. In chains of Thalassiosira, 

 several hundred cells in length, no difference in size between 

 individuals could be made out. Auxospores are commonly 

 formed with every species, but only in cultures of Coscino- 

 discus and Th alassiosira 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 auxo- 

 spores. What exactly is the fate of these auxospores, which 

 are often exceedingly numerous, we have not been able to 

 make out. It seems that cultural conditions are not favour- 

 able to this mode of reproduction, and that the auxospores do 

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

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

 frnstules must have been seen in some at least of the very 

 numerous samples examined. As it is, what has been seem 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 

 chromatophores 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 megafrustules were formed. In these the 

 chromatophores, etc., gradually formed into the shRpe of the 

 diatom (Coscinodiscus) ; the siliceous coat Avith plain 

 eccentric markings was easily seen inside the spore; and 

 lastly, the cell-wall of the spore burst, leaving the mega- 

 frustule free. The megafrustule was measured and found to 

 have a diameter three times that of the parent cell. 



In the case of the diatom w'e have very largely used for 

 feeding larvfe, etc., namely Nitzschia closterium, forma 



