37 
Studies on Cyanophyceae. 
majority of cases the cell-contents are distinctly granular, but under 
certain circumstances they may acquire a homogeneous character. 
The heterocysts (figs. 1, 2 and 4) are, as a rule, in the species 
under discussion, not much larger than the ordinary vegetative cells 
and in certain cases may even be below the average size. They 
develop from the ordinary vegetative cells and are mostly to be 
recognised at a very early stage. The cell-contents gradually 
become paler, whilst a bright, highly refractive granule appears, 
lying apposed to one or both of’ the end-walls of the cell (figs. 2, 8b 
and 9); these granules are quite evident at a time when the young 
heterocyst still shows an unthickened wall and a distinct blue-green 
colouration of its cell-contents, although, as will be shown presently, 
some considerable alteration has already taken place in them. 
Except for this terminal granule the contents of the heterocysts at 
this stage are homogeneous or very finely granular. One can very 
often observe, that in the cell adjoining the heterocyst, a similar 
granule lies in contact with the middle of the wall, adjacent to the 
heterocyst, being thus situated exactly opposite and in close 
proximity to the granule in the latter. This appearance is not 
confined to the heterocysts, for granules are often seen lying 
opposite one another in the same way in two adjoining vegetative 
cells. After the appearance of this terminal granule the cell-wall 
of the heterocyst becomes more or less thickened, the thickening 
apparently commencing in one or both of the end-walls, and the 
cell-contents become colourless; in all cases there appears to be a 
certain amount of contraction of the contents. Two kinds of 
heterocysts are to be distinguished: Terminal ones, which have a 
spherical shape, flattened on the side towards the vegetative cells, 
and intercalary ones, which are more or less barrel-shaped and 
flattened at each end. 1 The bright granule above mentioned, as 
lying apposed to the one (terminal, figs. 8b and 9) or both (intercalary, 
fig. 2) of the flat ends of the heterocysts is usually no longer repre¬ 
sented in later stages ; but similar granules, generally of large size, 
are often to be found, lying singly or several in number in the cavity 
of the heterocyst (fig. 9). 2 The end wall or walls generally develop 
two little lip-like prominences internally, one on each side of the 
pore, which thus becomes very distinct (figs. 1, 4 and 9). In old 
heterocysts a plate of thickening extends right across the pore of 
the heterocysts; under a high power this plate appears to have no 
1 These terms are also employed by Hansgirg (’87, p. 123.) 
* These granules in the cavity of the heteroC5’st are not very 
evident unless stains are employed (cf. below). 
