38 INTRODUCTION. 
cell of protoplasm surrounded by a layer of superficial cells; in other cases the 
furrows grow radially inward without intersecting till near the centre, thus form- 
ing narrow cones and pyramids with their bases outward (Fig. 14, D). 
With the progress of cleavage the contraction of the protoplasm in 
Synchitrium becomes very noticeable, the furrows open widely and 
the masses tend to become rounded. The cell is thus split up into a 
number of blocks of varying size and containing a variable number of 
nuclei. In these large cells or portions of protoplasm cleavage fur- 
rows show no tendency to orient themselves with reference to the 
nuclei, but as the process advances and the pieces become smaller the 
nuclei are seen to be more evenly distributed. Finally, the result is 
always the separation of the cytoplasm into uninucleate masses or 
cells (Fig. 14, F). 
It is interesting to note that the process which, in the beginning, 
seemed to be independent of the nuclei, is finally directed solely from 
the standpoint of their distribution. 
From this process of cleavage in Syxchztrzum it is ‘at once appar- 
ent that we have a method of cell-formation which is fundamentally 
different from either of the two methods described in the preceding 
pages. Here there are no kinoplasmic fibers developed in connection 
with the nuclei under whose instrumentality plasma membranes are 
formed, and, in earlier stages of cleavage in the sporangium, new 
plasma membranes seem to be developed independently of nuclei, 
though not in their absence. 
In certain cases of cell-formation by cleavage, in which very large 
multinucleate masses of protoplasm are involved, as in the plasmodium 
of certain Myxomycetes and in sporangia of such Phycomycetes as 
Pilobolus and Sporodinia, vacugles play a very important part either 
directly or indirectly. 
The first indication of the cleavage which is preparatory to the for- 
mation of the columella-wall in the sporangium of Pz/obolus (Harper, 
”99) is seen in the gradual appearance of a layer of vacuoles larger 
than the rest, and lying in the curved surface which marks the outline 
of the columella: , 
The vacuoles become flattened in their radial axes parallel to the surface of 
the sporangium, and form thus disk-like openings which tend to fuse at their 
edges. At the same time a circular cleft is seen to start from the edge of the 
sporangiophore opening . . . and to develop upward, cutting into the 
vacuoles, so that they become connected into a continuous furrow (Fig. 15, A). 
Whether this furrow is continued upward to enclose the whole dome-shaped 
columella, or whether the vacuoles in the upper portion fuse edge to edge before 
the cleft reaches them, is difficult to determine. The process is a progressive 
one, the cleavage being complete in certain portions sooner than in others, and 
