ZYGOSPORES. 
263 
creep about in troops, come into close contact with one another, and finally coalesce 
into large lumps. As soon as one of these lumps is formed, the rest collect from all 
sides round it as a centre, coalesce with it, and increase in this manner the mass of 
protoplasm which then becomes more and more rounded off. There is every reason 
to believe that this collective union of zoogonidia is a conjugation and therefore a 
sexual act, in the same sense as the conjugation of the zoogonidia of the Pando- 
rineae ; and the large mass of protoplasm formed in this way, which is called a Plasmo- 
dium, must therefore be treated as the analogue of the zygospore. The only difference 
is that in this case the zygospore does not become invested with a cell-wall nor go 
through a period of rest, but at once undergoes further development, becoming 
transformed into a stalked fructification which produces a large number of spores. 
In accordance with their mode of origin these spores may be compared not merely 
with the zoospores developed from the zygospore of Pandorina, but also with the 
ascopores of the Ascomycetes, and even with the spores of the Muscineae. The 
formation of this fructification out of the roundish plasmodium of DictyosteVmm com- 
mences with the production in its centre by free cell-formation of a number of cells 
each surrounded by a cell-wall of cellulose, which unite into a parenchymatous tissue 
forming in the interior of the Plasmo- 
dium a column or stalk standing erect 
on the substratum. As this column con- 
tinues to grow in height, the rest of the 
protoplasm which surrounds it creeps 
up it, and collects at its summit into 
a round lump, the entire substance of 
which now breaks up into a number of 
spores. This example furnishes the 
simplest case of the course of develop- 
ment of a Myxomycete. In most other 
instances it is much more complicated, 
the development being more complete, 
and a reduction to this plan becoming 
constantly more difficult. But the first 
stage of the development is essentially 
the same in all Myxomycetes. Each 
spore gives birth to from one to eight 
amoeboid bodies, which grow and multiply by repeated division, subsequently coalescing 
with one another in large numbers and producing plasmodia. The plasmodia of other 
Myxomycetes, however, do not at once produce fructifications, but maintain for a 
longer period an independent life, creeping about in the moist interstices of their 
substratum; as, for example, the yellow plasmodia inside a tan-heap, which at length 
come to the surface, and then coalesce into the large bodies which are known as 
' flowers of tan.' Other plasmodia creep about for a time on rotten wood or among 
decaying leaves, at length in the same way reaching the surface, and then usually 
producing simultaneously a number of fructifications. Fig. 172 ^ may give an idea of 
the mode in which net-like structures are produced by these motions of the plasmodia. 
The substance of the plasmodium is thin and granular in the interior and bounded on 
the outside by a homogeneous pellicle ; it is constantly changing its form ; protuberances 
arise at various spots which move onwards with a flowing and creeping motion, ramify, 
and anastomose with one another, while the substance flows into them from behind, 
and in this manner enables the entire structure to creep gradually forwards. Imme- 
diately before the period when the fructification is produced, a tendency is manifested 
to creep up erect bodies, so that finally the fructification is frequently found on 
plants, stems, or leaves at a considerable distance from the original nutrient sub- 
stratum. At this period the plasmodium collects at certain spots, and forms either 
Fig. 173. — Physarum albtnn (after Cienskowski). i spore ; 
2 escape of its contents ; 3 the contents when free ; 4, 5 the same in 
the form of a zoogonidium provided with a cilium ; 6, 7 after loss of 
the cilium ; 9— 11 coalescence of the amceboid bodies ; 12 a small 
Plasmodium. 
