SMITH: TETRADESMUS, A NEW COENOBIC ALGA 81 
protoplast contains a pyrenoid and one does not. This fact can 
be determined with the greatest accuracy since the staining re- 
action of the pyrenoid makes it the most conspicuous structure in 
the cell. Hundreds of cells at this stage of development have been 
observed, and in every case the pyrenoid was easily distinguished 
in one of the daughter protoplasts while the other was empty. 
At first this cleavage without a division of the pyrenoid was 
thought to be an abnormality, but the abundance of cells in this 
condition and the failure to find a single case in which a pyrenoid 
appeared in each of the daughter protoplasts show that this is the 
normal condition. 
The first plane of cleavage is usually approximately at right 
angles to the long axis of the cell (FIG. 10A), but soon after the 
cleavage the daughter protoplasts arrange themselves inside of 
the mother cell wall so that the original cleavage plane appears 
to be diagonal (FIG. 10B). The stimulus which leads to this 
twisting of one of the protoplasts upon the other is not known. A 
second division of the daughter nuclei now occurs. Fic. 11 shows 
this fully completed, the direction of the division, to judge by the 
position of the resting nuclei, always being parallel to the primary 
cleavage plane. There is some variation in the time at which this 
second division takes place; in some cases it occurs when the 
primary cleavage plane is only a few degrees from the transverse 
position, but in others the primary plane has come to lie at an 
angle of 45 degrees with its original position before the second 
nuclear division occurs. Fic. 11 shows that after. the second 
nuclear division a single pyrenoid is still in evidence in one of the 
protoplasts while there is none in the other. The amount of 
starch surrounding the pyrenoid is approximately the same as 
that in the undivided cell, but this is not brought out in the figures, 
since material which is well stained for the nuclear structures does 
not show the starch about the pyrenoid. Fic. 10B, from material 
which was stained especially to bring out the pyrenoid, shows that 
after the first cell cleavage there is still considerable starch about 
the pyrenoid. 
Following the second nuclear division there is a cleavage in each 
of the protoplasts. These cleavages seem to be usually simul- 
taneous, but FIG. 12 shows a case in which the cleavage has been 
