CH.VIII.— MORPHOLOGY AND COURSE OF DEVELOPMENT. —MYXOMPCETES. 427 
It consists of a sticky substance which swells in water and contracts in alcohol and 
is scarcely coloured by iodine, and must therefore be different from protoplasm. It 
passively follows the movements of the plasmodium. Portions of it often remain 
adhering in the form of thin films of mucilage to spots from which a plasmodium has 
moved away. This envelope is often very thin round the rapidly swelling extremities 
of the branches, and cannot be distinguished at all round slender pseudopodia, 
having been either pierced through by them or extenuated by their advance till it is 
no longer perceptible. 
The plasmodia of the Stemoniteae, Trichiaceae, Ceratieae and Lycogala have 
in the main the same structure and power of movement as those of the Physareae, but 
they never have the granules of calcium carbonate and therefore usually appear to be 
much more finely granular than in the Physareae. The dark-blue or violet-brown 
plasmodia of the Cribrariae and Dictydium contain large brown granules of some 
organic substance, but have as yet been very insufficiently examined. The plasmodia 
of Lycogala which live in rotten wood are surrounded by a thick colourless membrane; 
I observed a similar membrane some time ago in Arcyria punicea. Itis not yet ascer- 
tained how this membrane behaves in the movements. I was unable to see it in former 
years in specimens of Lycogala grown in water. I found the plasmodia of Stemonites 
fusca, when it issues from the substratum to form its sporangia, surrounded by a stout 
envelope, the inner and thicker layer of which is‘coloured a dark blue by iodine 
while an outer thin layer remains colourless. All the plasmodia last mentioned are 
inconspicuous bodies, the stouter branches of which in Arcyria punicea are not more 
than 16 a in thickness, in Lycogala not more than 24 ». They live for the most part in 
the interior of rotten parts of plants, especially rotten wood, and are not visible to 
the naked eye till they come to the surface to form sporangia. 
Section CXX. Transitory resting-states. Those stages of the development 
in Myxomycetes which have the power of motion are able to pass into resting-states 
and to return again under favourable conditions to the state of movement. Three 
resting-forms are at present known: microcysis, thick-walled cysts (Cienkowski) and 
sclerolia. 
It appears from cultures of Chondrioderma difforme that these transitory 
resting-states are not necessary members of the course of development. Their 
formation would seem to take place only when the development of the swarm-cells into 
plasmodia or of the plasmodia into sporangia is interrupted by insufficiency of food, 
by slow desiccation, or by slow cooling to below a certain minimum. But there are 
a number of observations which also point to other at present unknown causes. 
The state of movement is restored when the bodies after desiccation are again placed 
in water of the proper temperature. 
The term microcyst was given by Cienkowski to the resting-state of the swarm- 
cells. Under the conditions above mentioned these cells assume the form of spheres 
which are smaller than the spores and are surrounded by a very delicate colourless 
membrane, as in Perichaena liceoides according to Cienkowski, or are without a 
membrane but provided with a very firm marginal layer. In other respects their 
structure remains the same as that of motile swarm-cells, only that the vacuoles in 
many cases disappear and the protoplasm becomes more dense. The swarm-cells of 
Didymium praecox and D. difforme encysted in this way and perfectly dry retain 
