180 bulletin: museum of comparative zoology. 



visible ; tlie girdle part of the cell-wall is very narrow. When the cells 

 begin to divide, the two valves diverge, and a large girdle part becomes 

 formed, as shown iu Plate 2, fig. 20. This girdle part has no structure 

 at all ; it is only a very thin, hyaline, silicious tube or cylinder, as can 

 be clearly seen in ignified specimens mounted in styrax balsam, where 

 the transparent girdle is contrasted with the porulate valves. As Plate 

 2, fig. 21 shows, the plasmatic contents are much altered at this stage ; 

 the plasma lies close to the girdle wall ; and must be adherent to the in- 

 ner side of it, as it has not withdrawn in the preservation. The nucleus 

 is large and of an amoeboid shape, and nearly all the chromatophores 

 are aggregated in the parts of the plasma nearest to the disc. A later 

 stage has been given in Plate 2, fig. 22. The cell-division is now accom- 

 plished, and the cell-wall is nearly everywhere as thick as usual, the 

 discs only being somewhat thinner, but the plasma has not reached its 

 final condition. It lies close to the newly formed disc-walls, and only 

 few of the chromatophores have changed their place, the main part re- 

 maining aggregated iu the part of the plasma most remote from the new 

 cell-wall, and here we find also the nuclei. Some time later the initial 

 stage (Plate 2, fig. 19) is again reached. I think that the manner of 

 cell-division here described — and this is, I believe, the normal manner 

 in this group of Melosirae — has the effect that the diminution of size, so 

 characteristic for the Diatoms, is reduced to a minimum in spite of the 

 very thick cell-wall of these species. It would be an interesting matter 

 to follow the process more closely on fresh material and on material 

 fixed for cytological purposes. 



C Myxophyceae and Chlorophyceae. 



"With regard to the Blue-green and the Green Algae very little is to be 

 said, as they play so small a part iu the February samples. It is sufficient 

 to refer to the papers by W. Schmidle (1898), G. S. West (1907), and 

 myself (1908), as they are based upon much better material of these 

 Algae. I liave only two small points to add, both due to examination of 

 a sample of phytoplankton from Lago di Muzzuno iu Tessin (.Switzerland) 

 collected by me on July 20, 1908.^ The pliytoplankton of Lago di 

 Muzzano was very rich in Green Algae. Among the many species ob- 

 served was a form of Pediasb-um horyanum which agreed exactly with 

 var. rug2iJosum described by West (l. c, p. 136, pi. 5, figs. 8-9) from 

 Victoria Nyanza j this form is then not endemic for Victoria Nyanzi, 



1 Water temperature 20° C. 



