S lurch.- — Harvey ella mirabilis (Schmitz & Reinke). 87 
tapering almost to a point, while the cells which follow it 
become larger and more rounded, in proportion as they are 
further removed from the distal cell. The walls between 
the Rhodomela - cells appear to become gradually swollen as 
the parasitic filaments force their way into them. 
The parasite having reached the exterior of the host, rapidly 
develops branched filaments, usually consisting at this stage 
of rather elongated cells, which, however, soon become more 
full of contents and assume an oval or almost spherical shape. 
It is at this time that the first indications of a distinct 
peripheral layer are seen. The outermost cells of the frond 
are closely packed in a 1- or 2-celled layer ; these cells 
are somewhat more regular in shape than the other cells 
of the parasite, being oblong with rounded ends. They 
stain more rapidly and deeply with Hoffmann’s blue than 
the remaining cells. At this stage the swellings on the 
Rhodomela - stems have a greyish-white appearance. 
In the meantime the Harvey ella - filaments in the interior 
of the host have continued to force their way among the 
large central and pericentral cells until they occupy a space 
as large or larger than the mature external portion. Some- 
times these internal filaments branch enormously, and form 
large fan-shaped masses of radiating filaments (Fig. 3). As 
these filaments grow they gradually absorb the contents 
of the host-cells which they surround. Tn Fig. 3 some of 
the host-cells have their contents partially absorbed, while 
others are quite empty, only their walls remaining. The 
external portion of the parasite now rapidly increases in size, 
owing to the growth and division of the distal cells of the 
peripheral chains. These growing cells divide in two ways. 
They are divided into two parts by a curved transverse wall, 
the lower half gradually increasing in size and passing over 
into the ordinary tissue of the frond, the upper half rapidly 
elongating and dividing again in a similar way. But fre- 
quently the distal cell is divided by a curved, oblique, longi- 
tudinal wall, producing in the ordinary way a sub-dichoto- 
mously branched filament. As the frond increases in size, 
