20 



DIATOME.E. 



toplasmic masses (gametes) coalesce to form a zygote, the cell- 

 nuclei and chromatophores also fusing together. The zygote in- 

 creases in size, and surrounds itself with a firm, smooth, siliceous 

 wall the perizonium. The auxospores, whichever way they arise, 

 are not resting stages. The germination of the zygote com- 

 mences by the protoplasm withdrawing itself slightly from the 

 cell-wall and constructing first the larger valve, and later on 

 the smaller one ; finally the membrane of the zygote bursts (e.g. 

 Himantidium). 2. The conjugation occurs in a similar manner, 

 but the protoplasm of the cells divides transversely before 

 conjugation into two daughter-cells. Those lying opposite one 

 another conjugate (Fig. 16) and form two zygotes. The for- 

 mation of the perizonium, and germination take place as in the 

 preceding instance (e.g. Epithemia). 3. Two cells place them- 

 selves parallel to each other, and each of the two cell-contents, 



FIG. 16. Conjugation of Cymbella variabilis. A, The protoplasm in the two cells has 

 divided into two masses ; B these masses coalesce in pairs; the cells (B C) enclosed in a 

 mucilaginous matrix. C D Auxospores and their formation. 



without coalescing, becomes an auxospore. The formation of 

 the wall takes place as in the preceding case. This is found in 

 the Naviculea3, Cymbelleae, the Gomphonemeae (e.g. Frustulia, 

 Cocconema). 



The Diatomacece may be found in salt as well as in fresh water 

 (often in such masses that the colour of the water or mud becomes 

 yellow or brown ; in the same manner the genera Chcetoceros, 

 Rhizosolenia, Coscinodiscus, and several others, form large slime- 

 masses, " Plankton " on the surface of the sea), on damp soil and 

 in dust blown by the wind. They occur as fossils in the recent 

 formations, often in large deposits (siliceous earth, mountain 

 meal), as in the cement lime in Jutland, the alluvial deposits 

 beneath Berlin, in clay strata beneath peat bogs, in guano, etc. 



