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 Synchitrium 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, vacuoles 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 Pilobolus (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 



