258 XX. STRUCTURE OF THE THALLOPHYTA. 



valves are inserted within the two outer valves. After division of 

 the contents of the mother -cell, these inner halves of the wall are 

 developed for each individual. Each cell, therefore, has an older 

 and a younger half of its wall, i.e., one valve, the outer one, 

 belonged to the mother-cell, and the other, inner valve, is peculiar 

 to the present individual ; and further consideration shows that 

 the difference of age between the two valves may thus be very 

 considerable. 



Movements of Diatoms. The Pinnularia cells are motile, 

 They usually progress in the direction of their long axis, either 

 uniformly, or by jerks, also turning off now and then laterally from 

 their path. As a rule they creep upon some substratum, but can, 

 however, swim free in water. The mode of movement is still a 

 matter of controversy. 



Siliceous Skeleton of Diatoms. We now place a preparation 

 of Pinnularia on a plate of mica, and heat it over a gas or spirit 

 flame. We then lay the plate of mica, when cold, upon our 

 object-slide, and observe the preparation dry, but under a cover- 

 glass, with strong magnifying power. We can see that the Pin- 

 nularia remain as perfect skeletons. With short heating they 

 become somewhat brown, from the carbonised organic substances ; 

 with longer continued heating they are colourless. Hydrochloric 

 acid does not touch them ; they consist of silica, and retain and 

 show the finest peculiarities of the cell- wall, which must there- 

 fore have been silicified in a high degree. The furrows show in 

 this preparation very clearly as dark striae, and they are extremely 

 good for studying other structural peculiarities of the wall. 

 Especially beautifully visible in valve- view are the clefts, which 

 run on both sides from the median nodules to the terminal 

 nodules. Their broadening at mid-length is manifest. In the 

 girdle-view the edges of the two halves of the cell-wall show 

 clearly ; moreover, on the overlapping parts are seen two lines 

 parallel with one another and with the edges of the valves, which 

 do not extend to the ends of the cell. Flint-skeletons quite as 

 beautiful as these are also obtained if we first allow a drop of 

 concentrated sulphuric acid to act upon our diatoms, and after 

 some time add 20 per cent., and then gradually concentrated 

 chromic acid, and finally remove these reagents with water. 

 Diatom valves which are poor in silica will neither bear heating 

 red-hot, nor this last method of procedure ; they must instead be 

 laid for from four to seven days in hydrochloric acid, to which a 



