496 Harper. — Cell-Division in Sporangia and Asci. 
columella wall. The radiating strands from the upper edge 
of the ridge are shorter as the distance from the edge of the 
ridge to the dense spore-plasma is less. By midnight the 
layer of spore-plasma reaches its definitive thickness. The 
dome-shaped columella cavity is still a large cell-sap cavity, 
and the inner surface of the dense spore-plasma passes 
gradually into this by means of a spongy layer which is 
rapidly becoming typically vacuolar as the streaming ceases. 
In the outer portion we can now roughly distinguish three 
parts in the protoplasm of the sporangium ; the outer dense 
spore-plasma, passing into a spongy layer on its inner surface, 
and the central cavity of the columella. In the outer part of 
the spongy layer the split between the spore-plasma and the 
columella-plasma is to be formed. The first indication of this 
cleavage 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 be- 
come 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 
just outside the protoplasmic ridge above noted, and to 
develop upward, cutting into the vacuoles so that they become 
connected into a continuous furrow (Fig. 12 ). 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 
at a very late period strands of protoplasm are seen connect- 
ing the spore-plasma with that in the columella. It is not 
impossible that many of the apparently disk-shaped vacuoles 
are sections of curved openings which burrow through the 
plasma from below upwards. Frequently vacuoles which are 
distinct in one plane are seen, by focusing up or down, to be 
connected. There can be little doubt however that a con- 
siderable part of the cleavage of the columella is accomplished 
