Hazen: Lire History oF SPHAERELLA LACUSTRIS 221 
parts, each of which becomes invested with a cellulose wall; each 
of these daughter-cells in turn forms two cells, all four become 
clothed with cell-walls and remain enclosed in the enlarged mem- 
brane of the original cell; each of these four cells may divide 
again to produce a third generation of daughter-cells which may 
become motile. In a later supplementary description, Cohn (’54) 
illustrates the development of Ch/amydococcus by figures showing 
the formation of the zooids in a manner esssentially like that which 
I have observed (Figs. 2—5, 46-48), that is by successive divisions 
without the formation of cell-walls in the intermediate stages. 
Later still Cohn * said there was need of further proof of the 
development. of resting-cells directly from resting-cells (Se/dstthet- 
/ung) which he had first described. 
Braun ('51) asserted that a vegetative division does take place, 
but not as a part of the regular cycle of development previously 
described. He found that where the cells are kept simply moist 
and exposed to the air, as is the case under natural conditions, es- 
pecially in the milder intervals of winter and on the moist edges 
of rock basins at other seasons, the cells multiply either by simple 
division or by double halving, but the daughter-cells thus formed 
do not slip out of the mother-cell-wall; they gradually acquire 
thick, close cell-walls, while the mother cell-wall expands and dis- 
appears. By the frequent repetition of such a process masses of 
cells become pressed together so that thick crusts of cells bounded. 
by flat surfaces may be formed. This vegetative division is very 
rarely found in cultures in the laboratory, because of the difficulty 
of producing artificially the conditions which foster it. 
Cohn ('50) notes that temperature is one of the factors which 
determine whether resting or motile cells shall be produced. On 
one occasion I found in a glass jar kept on a window ledge a 
large number of red cells which had developed daughter-cells in- 
distinguishable from ordinary zooids in form. None of them, 
however, became motile (Fig. 49) and the only possible explana- 
tion was that after division had begun a sudden drop in the tem- 
perature prevented the daughter-cells from becoming zooids. 
I have frequently examined the Sphaerella growing on a moist 
.. * Cohn and Wichura. Ueber Stephanosphaera pluvialis. Nev. Act. Acad. Caes. 
26!: Nachtrag. 23. 1857. 
