40 INTRODUCTION. 
and thus enclose all the protoplasm in the sporangium, is an interesting ques- 
tion. The necessity is evident that the cleavage should proceed through a 
tolerably dense plasma, and this is, perhaps, due to the need of two proto- 
plasmic surfaces in contact in order to form a cell-wall. 
The fact that the columella is not deposited on the surface of the 
central vacuole seems to indicate that the limiting layer of a vacuole 
is not quite a plasma membrane, although it may partake partly of 
the real nature of one. Although there is much to show that the wall 
of a vacuole, such as we are dealing with here, and a plasma mem. 
brane are closely related, yet the author is not quite ready to admit 
that they are the same, Why two plasma membranes should be in 
contact in order to form a cell-wall, as suggested by Harper, is not 
quite clear to the author, since in many cases a single plasma mem- 
brane will secrete a cell-wall. 
In the cleavage of the spore-plasma, which begins soon after the 
columella is complete, vacuoles also take an important part. The 
cytoplasm becomes somewhat vacuolar, and the numerous nuclei are 
rather evenly distributed throughout its mass. Cleavage furrows 
appear now near the base of the sporangium, cutting the surface into 
irregular polygonal areas (Fig. 15, B). At the same time vacuoles 
in the interior become angular, appearing three-cornered in section, 
and their edges cut through the cytoplasm to meet similar cleavage 
furrows from adjacent vacuoles (Fig. 15, B). In the meantime the 
surface furrows which have been growing deeper meet and become 
continuous with the edges of the vacuoles. By pressure of the adja- 
cent plasma-masses, the surfaces of the vacuoles which were formerly 
convex become concave, and the vacuoles appear as intercellular 
spaces between the cleavage-segments. In this manner the spore- 
plasma is marked out into irregular blocks, apparently without refer- 
ence to the size or number of nuclei they contain. A continuation of 
the process cuts the spore-plasma into oblong rounded sausage-shaped 
masses containing generally two to four nuclei in a row (Fig. 15, C). 
These oblong masses now divide transversely to form rounded bodies 
with one or few nuclei (Fig. 15, D). This completes the primary 
cleavage by which the spore-plasma has been cut up into smaller 
units with one or few nuclei, These units are not the spores. They 
undergo a period of growth and nuclear division before the final 
cleavage divisions take place by which the mature spores are pro- 
duced. The last divisions are, however, similar to the first, presenting 
the simpler process of cleavage or fission. 
In the sporangium of Pilobolus, we have a cleavage which is of 
the same type as in Synchitrium, with the exception of the promi- 
