86 S lurch. — Harvey ella mirabilis ( Schmitz & Reinke). 
Rhodomela subfusca , but after March it becomes rarer, and 
I have never been able to find any specimens during July, 
August, or September. At the beginning of October certain 
stems of Rhodomela subfusca were noticed to be slightly 
swollen at various places, and on cutting sections of these 
swellings, it was found that a considerable development of 
Harveyella - filaments had taken place in the interior of the 
host ; these filaments were growing between the cells of 
the Rhodomela , but none of them had yet reached its external 
cell-layer. The swollen appearance was caused by a great 
increase in number of the cortical cells of the Rhodomela . 
The probable course of the development of the parasite is 
as follows. After the spores are set free, from January 
to March, they in some manner make their way between 
the cortical cells of a Rhodomela , and then commence to 
send filaments between these cortical cells toward the large 
central cells of the host, as also among the cortical cells 
themselves : this is the earliest stage I have seen, and it 
is represented in Fig. i. At this stage there is no external 
swelling of the Rhodomela- stems. Soon afterwards a con- 
siderable increase in the number of the cortical cells of 
the Rhodomela takes place, producing the swollen appearance 
(Fig. 2), and at the same time the filaments of the parasite 
grow rapidly between the large pericentral cells of the host 
(Fig. 2), where they are much branched and composed of 
well-developed cells. Fewer filaments, consisting of, as a 
rule, thinner cells, grow between the numerous small cortical 
cells which cause the external swellings of the Rhodomela - 
stems ; these latter filaments eventually reach the surface 
(Fig. 2). Just beneath the external membrane of the Rho- 
domela these Harveyella -filaments become much branched, 
and give rise to a closely-packed layer of cells. At about 
this time the external membrane of the Rhodomela is broken 
and its place taken by the gelatinous external membrane 
of the Harveyella. A portion of one of the Harveyella- 
fi laments which grow between the cells of the host is drawn 
in Fig. 18. The distal cell of the filament is very narrow, 
