on species of Tolypothrix. 651 
almost spherical (Fig. 2,6). Usually the constrictions, and 
therefore also the transverse walls of the Fungus correspond 
in position with the dividing septa of the algal filament ; this 
is due to the narrowing down of the fungal hypha, when it has 
to pass through a wall of the Alga. Not infrequently the 
Fungus branches within the host (Figs. 24, 25) and a very 
complicated tangle may sometimes be formed in this way 
(Fig. 26). 
The contents of the fungal segments are colourless, and con- 
sist of vacuolar protoplasm with one or more bright granules 
of some oily substance. I Was not able to make out nuclei. 
The effect on the cell-contents of the Alga was in the first 
place decolourizing. It would appear that the action of the 
Fungus does away with the special colouring-matter of the 
Tolypotkrix-ceUs, so that those which have been recently 
attacked have a dirty yellowish-green colour. But even this 
soon disappears, and ultimately the entire cell-contents, as 
well as the dividing-walls of the Alga, are dissolved away. 
In the earlier stages of this process the contracted protoplasm 
forms a kind of granular sack round the fungal cell (Figs. 
19, 20). 
The most striking point about this Fungus, and a point 
in which it differs from the species to be described below, 
is the frequent occurrence of thick-walled dark brown cells in 
the course of the parasitic hyphae ; such cells are to be seen 
in all my figures of this species. These spores were formed in 
considerable numbers in the course of each hypha ; they were 
most commonly single (Figs. 21, 24), often in twos (Figs. 
22, 23), and sometimes aggregated together in large numbers. 
Apparently any cell of the Fungus could develop into one of 
these spores, but there seemed a great tendency for their 
formation inside the heterocysts of the Alga (Fig. 20), prob- 
ably because the conditions of nourishment are worse there 
than in the other cells of the filament. The heterocysts 
present a considerable obstruction to the passage of the 
Fungus, and in many cases the latter was observed to ter- 
minate at these points (Fig. 19) ; sometimes these terminations 
