424 SECOND PART.—MFCETOZOA. 
become less frequent, most of the swarm-cells have the creeping form in which they are 
without cilia, and many have increased in size and contain single large strongly 
refringent granules. Then they approach close to one another in groups com- 
pösed of two or more and again separate, till at length two or three are seen to come 
into close contact with one another and become fused together into a single body, 
the young plasmodium. This body exercises an attraction in a way not yet 
explained upon other swarm-cells of the same species, and they attach themselves one 
by one to its surface and coalesce with it. The newly formed plasmodium is 
distinguished by its greater size from the swarm cells without cilia, while it exhibits 
essentially the same movements and changes of shape. 
The plasmodia when once formed by the coalescence of the swarm-cells begin 
to increase in size, and as they grow assume the form usually of branched strands, 
their dimensions far exceeding those of the swarm-cells (Fig. 183, 12, and Fig. 185). 
The latter is especially the case in some of the Physareae (Calcareae of Rostafinski) ; 
the stouter branches may in these species even exceed the thickness of a strong bristle, 
and the plasmodium in the form of a copiously branched reticulated or frill-like 
expansion covers surfaces varying in extent from an inch to a foot. This is the case 
in Fuligo varians (Aethalium septicum of authors), Leocarpus vernicosus, Didymium 
Serpula and D. praecox and Diachea elegans. Other species of Physareae as 
Chondrioderma difforme and Didymium leucopus, Fr. have usually much smaller 
plasmodia, just large enough to be clearly seen with the naked eye or invisible to it, 
and this is the case with all the other forms which have been hitherto examined not 
belonging to the Physareae. 
The plasmodia of the Physareae if sufficiently supplied with water spread over 
the surface of the substratum, usually decaying parts of plants, in the form of veins and 
net-works of Veins which have been well known since Micheli’s time and especially from 
Fries’ excellent descriptions, and from their mesentery-like appearance have been termed 
Mesentericae. As they spread also readily on the microscopic slide, their structure 
and conformation have been specially investigated (Figs. 184, 185). ‘They are chiefly 
composed of a soft protoplasm of the consistence of cream, which may be readily 
spread out into a shapeless smear and is usually colourless but sometimes yellow, as 
in Fuligo, Leocarpus vernicosus, and Didymium Serpula, or a reddish yellow as in 
Physarum psittacinum. The microscope enables us to distinguish in them a colour- 
less homogeneous slightly turbid fundamental hyaloplasm which usually appears 
distinct as a bounding layer of varying breadth, and a granular protoplasm which 
forms the chief mass of the plasmodium. Vacuoles are formed not unfrequently in 
the granular plasm and sometimes also in the hyaloplasm, some of which alternately 
disappear and reappear while others are more stable. The granules are more or less 
numerous in different species and individuals, but are always present in sufficient 
quantities to make the great mass of the plasmodium highly turbid, and in the stouter 
branches of some species (Fuligo) quite opaque. The composition of the granules 
is in some cases not precisely determinable; in others they consist of calcium 
carbonate. These granules of lime form the larger part of the granular mass in the 
‘Physareae, where they are spherical with dark contours, glistening, and of tolerably 
uniform size. Where the yellow colouring matter occurs it generally accompanies 
the granules of lime, partly appearing as a thin coating on the single granules, partly 
