DESCRIPTION OF PLATES. 141 
contains ‘ vacuoles,’ occupied only by a clear aque- 
ous fluid, which are sometimes so numerous as to 
take in a large part of the cavity of the cell, so that 
the colored contents seem only like a deposit on its 
walls. 
“The flagella pass through the cellulose envelope, 
which invests their base with a sort of sheath; and 
in the portion which is within this sheath no move- 
ment is seen. During the active life of the ‘mo- 
tile’ cells, the vibration of these flagella is so rapid 
‘that it can be recognized only by the currents it 
produces in the water, through which the cells are 
quickly propelled; but when the motion becomes 
slacker, the filaments themselves are readily distin- 
guishable, and they may be made more obvious by 
the addition of iodine. 
“The multiplication of these ‘ motile’ cells may 
take place in various modes, giving rise to a great 
variety of appearances. Sometimes they undergo 
a regular binary subdivision, whereby a pair of 
motile cells is produced, each resembling its single 
predecessor in possessing the cellulose investment, 
the transparent beak, and the vibratile filaments, 
before the dissolution of the original investment. 
Sometimes, again, the contents of the primordial 
cell undergo a segmentation in the first instance 
into four divisions,” etc. 
The different appearances presented by these 
unicellular plants during different stages of their 
existence led the earlier observers to mistake the 
different forms for independent species, and the 
motile forms were very commonly regarded as ani- 
