CLEAVAGE IN DIDYMIUM MELANOSPERMUM (PERS.) MACBR. 1 39 
the last stages of cleavage in Didymium I have observed no such 
condensation of the spore plasm around the nuclei as one finds in 
Fuligo. The cleavage progresses in the same fashion until we have the 
condition shown in figure 17, in which both the outer surface of the spore 
mass and the inner surface have been cut up into small irregular 
protoplasmic blocks, while the middle region of the spore mass still 
shows large angular blocks of protoplasm which are being rapidly cut 
into by furrows over their entire surface. The ultimate result of this 
progressive furrowing is the formation of uninucleated rounded spores 
which are of fairly constant size, as shown in figure 18. They lie packed 
between the capillitial threads which still hold their radial direction 
running from the central columella to the peridium. • The capillitial 
threads at this stage are so brittle that they bjeak up to a considerable 
•extent in sectioning. 
As described, the segments cut off by the cleavage furrows in the 
■earlier stages, here as in Synchytrium, Pilobolus, Fuligo, and many 
other forms, are multinucleated and of very varying form and size. 
As the cleavage progresses, however, it is apparent that the distri- 
bution of the nuclei, though irregular and superficially considered 
accidental, is none the less in so far definite that the cleavage furrows 
never cut off non-nucleated segments and in the end each spore con- 
tains a single nucleus. The final cleavage stages here, as I have de- 
scribed in detail for Fuligo, always cut two-nucleated blocks into one- 
nucleated spores. These are the definitive spores of the slime mould. 
They show no subsequent growth or nuclear division in the process of 
ripening. 
The appearance of the cleavage figures suggests most strikingly 
the contraction which accompanies the process. In the period of 
most active segmentation the blocks of protoplasm lie quite separate 
from each other. Part of the open space as shown in the figures is 
doubtless due to shrinkage in fixation and in particular regions some 
segments have dropped out of the section. Still there can be no 
question that the free space between the blocks is to a considerable 
degree the result of the same extrusion of cell sap which led to the 
opening up of the broad surface furrows in earlier stages. 
Rothert (27) long ago observed the escape of cell sap and accom- 
panying shrinkage during spore formation in Saprolegnia. The con- 
ditions in Didymium with its radially placed capillitial threads on 
which in the early stages the extruded liquid accumulates in bead-like 
