194 SEAWEEDS 
former ones, it necessarily follows that every suc- 
ceeding generation becomes smaller by just as much 
as the thickness of the membrane at the girdle, since, 
once formed, the Diatoms do not subsequently in- 
crease in size. The character of size frequently used 
by certain students of Diatoms must therefore be 
fallacious, and no doubt the inordinate multipli- 
cation of species in this group is partly due to 
its use. 
When by this process, repeatedly carried out, a 
certain minimum diminution of size is reached, the 
original stature is regained by the formation of an 
auxospore. This may be formed in one of five 
different ways. (1) In the most simple form the 
valves open and the contents (generally, but not 
always, protected by a gelatinous envelope) emerge 
and increase in volume. ‘The auxospore so formed 
soon acquires a thin membrane which is not silicified 
(the perizontum). Two siliceous valves are then 
formed, first one and then the other, within this 
membrane, enclosing all the contents, and it is note- 
worthy that these first valves, while possessing all 
the ordinary characters of the species, exhibit slight 
differences in the membrane from that of the 
daughter-cells, subsequently produced by division 
from this parent form in the manner described. (2) 
Instead of one auxospore, two may be formed by the 
division of the contents of the cell, and these two 
naked cells emerge from the separated valves and 
develop each into an auxospore. (3) An auxospore 
may be formed after conjugation. Two Diatoms 
secrete a common gelatinous envelope; the contents 
