PHYSARELLA MIRABILIS PECK AND STEMONITIS FUSCA ROTH 277 
lime knots, as well as a branching thread. Figure 5, from an older 
sporangium, shows again the connection of thread and exterior. The 
hollow nature of the thread is here again plainly apparent where it 
widens out slightly. In this figure the protoplasm appears not at all 
shrunken, being in intimate contact with both capillitial thread and 
sporangial wall. 
Figure 6 represents a small section of material collected in the 
morning shortly before the time of complete maturity of the sporangia. 
In some sporangia of the collection at this hour, cleavage had pro- 
gressed to a considerable extent; while in the majority of cases 
cleavage is not so evident, the protoplasm being still in close contact 
with the thread, in the manner shown in the figure. Figure 7 does 
indeed indicate a case at this stage in which there is a slight space 
between the thread and the plasma membrane, and similar cases may 
in fact be found at other stages. I have interpreted such cases, when 
occurring at some time prior to cleavage, as arising from a slight shrink- 
age of the protoplasm away from the threads, due probably to fixation. 
At any rate, this earlier shrinkage should be clearly distinguished from 
the normal cleavage furrows which later follow the capillitial threads 
and which have been interpreted below as normal occurrences. The 
nuclei present an interesting appearance at this stage for, as is shown 
by one in figure 6, many are surrounded by a vacuole-like space. 
Harper points out ('146, p. 133) this same phenomenon in Didymium; 
and interprets it as ''hardly due to shrinkage in fixation, since they are 
scattered among other nuclei which show no such peculiarity." 
Capillitium in Relation to Cleavage in Physarella 
Although the threads may be found in contact with the cytoplasm 
until a short time prior to cleavage, at the first suggestion of that phe- 
nomenon there is no doubt but that this contact ceases to exist. 
Harper has elaborated this point in his Didymium paper. Figure 8 
shows an early stage of cleavage, in which every forming furrow is 
plainly associated with one or more capillitial threads, which appear in 
cross or oblique section in the preparation. The threads all appear 
to be hollow; that this appearance is not an artifact is attested by 
numerous observations on other material. At the upper right in 
figure 8 is a thread which has not yet been surrounded by a cleavage 
furrow. However, a slight space, as is commonly found at this stage, 
surrounds the thread. Conditions in the left part of figure 8 portray 
