Life-history of Anabaena Cycadeae. 373 
in some cases a central granule, respond similarly, but the old heterocysts 
are completely unaffected. 
Vegetative Division. 
The first indication of a division in a vegetative cell is the appearance 
of an indentation near the middle of the lateral walls (Fig. 9, b). This con- 
striction commences in the cell sheath, and extends to the inner investment. 
A very thin colourless area in the cell contents becomes visible, near the 
centre of the mother-cell (Fig. 9, c ). Wager says the division of a cyano- 
phyceous cell is brought about by the formation of a transverse septum, 
which grows inwards from the lateral walls, dividing the cytoplasm and 
nucleus into comparatively equal parts. Kohl describes a distinct polar 
separation of chromatic substances accompanied by the formation of a 
definite chromatic figure. Gardner, more recently, says that in the blue- 
green Algae are exhibited a series of nuclear structures passing gradually 
from a scarcely differentiated form of nucleus in which direct division occurs 
to a complex form showing a primitive type of mitosis. The division of 
the chromatin may precede or accompany the ingrowth of the cell-wall. 
In Anabaena Cycadeae the central body is a simple structure which divides 
directly after the new wall has begun to form ; then a colourless strip 
appears between the two central bodies, and gradually extends across the 
cell contents to meet the investment, thus developing into the intercellular 
colourless area (Fig. 9, d). The new cells separate, and the individual 
cylindrical sheath of each becomes evident (Fig. 9, e). 
Heterocysts. 
Heterocysts are always abundant, but particularly in old material and 
fully developed tubercles gathered in autumn. They vary somewhat in 
size, but are usually a little larger than the vegetative cells (Figs. 3, 10). 
They are formed from vegetative cells, their contents gradually becom- 
ing paler and more homogeneous. A bright highly refractive granule then 
appears near one, and later both, of the end walls of the cell (Fig. 10). 
These are cyanophycin granules, and frequently an exactly similar granule 
appears in the adjacent cell, on the side nearest the heterocyst (Fig. 10). 
Thickening of the cell-wall begins in one or both of the end walls, and 
extends laterally until the whole wall is thickened (Fig. 11) ; at the same 
time there is a chemical change, and the heterocyst walls exhibit definite 
cellulose reaction. The walls adjoining the vegetative cells develop two 
little lip-like prominences internally, one on either side of the pore (Fig. 12), 
which in old heterocysts may meet and form a plate across the pore. The 
granules disappear, the contentsbecoming colourless and contracting, but they 
remain attached to the wall in connexion with a vegetative cell until the 
