HISTORY OF DIATOMS. 



l 9 



or sporanges of Mdosira varians. Hitherto I have not been able to observe 

 them during their evolution. 



" In the case of Cxclotella comta the protoplasm, whilst increasing in 

 volume, separates the two cylindrical valves fitting into one another, and 

 oozes out on to the exterior in the form of a fairly regular discoid mass, the 

 axis of which is perpendicular to that of the small frustule. The membrane 

 containing the protoplasm then silicifies, and after deduplicating several 

 times the newly formed Cyclotella becomes perfectly circular. 



" Nitzschia palea is even more adapted than the preceding species for 

 studying the re-establishment of the maximum form of diatoms. 



" When the naviculoid frustules of this species have decreased in size to 

 from 28/x to 30//, the protoplasm which they enclose is considerably distended, 

 especially in the neighbourhood of the nucleus ; under the action of this 

 increase of volume the valves are forced open, nearly always at one end only, 

 and the external thalle of the diatom is rent to make way for the contents of 

 the cell, which increases in bulk and is lengthened in the direction of the major 

 axis of the Nitzschia and on both sides opposite the nucleus (fig. 12). 

 This yellowish body from 65/u, to 70/x in length, containing oil globules and 

 possessing the endochrome plates of a Nitzschia, is very irregular in form ; it 

 has the appearance of a cylinder rounded at the ends and inflated in the 

 centre; it may be torous, constricted, arcuate, extremely sigmoid, etc., 

 (fig. 13) but curiously enough it is animated, and from the time of its maturity 



(1 



fl 



II 



(i 



K 



I i 



1/ 



8 



10 



Fig. 12. 



