482 Harper . — Cell-Division in Sporangia and Asci. 
by additions along their inner margins till they intersect, and 
thus divide the protoplasm into irregular blocks or sometimes 
pyramids with their bases in the surface of the initial cell. (See 
Figs. 1, 2, 3, which are median sections through an initial cell 
in which cleavage has just begun.) Only at the very periphery 
the separation of the cut surfaces of the protoplasm to form 
a shallow notch, as it appears in section, reveals the true 
nature of the process as a pushing in of the free surface to 
form a deep though extremely narrow constriction. 
In many cases at first there is no separation of the newly- 
formed surfaces ; they remain closely appressed, up to the 
periphery of the cell. The groove appears in section, merely 
as a single line which the Zeiss apochromatic lens 140 ap. 
fails to resolve into two closely appressed surfaces (Fig. 3). 
The position of the line is further emphasized by the arrange- 
ment of the vacuoles, which are pushed aside and form in 
section two more or less regular rows in the plane of the 
newly-formed surfaces on each side of the furrow (Fig. 3). 
Such a line might be taken for a cell-plate which subsequently 
splits to form the boundaries of the protoplasmic segments 
or which is metamorphosed into the cellulose walls of the 
spores. That this line, however, in reality represents from 
the start two closely appressed surfaces is abundantly shown 
in many cases. Shrinkage in volume of the protoplasm goes 
on during the early stages of cleavage. This shrinkage is 
probably associated with the throwing off of water which has 
been seen to accompany sporangium-formation in many 
Zygomycetes. As a result of this shrinkage we frequently 
find that the cleavage of the protoplasmic mass is followed 
at once by the separation of the newly-formed surfaces, so 
that open furrows appear. This is regularly the case in 
Pilobolus , as we shall see later. There can be no doubt that 
the processes in forming the broad open furrows and the 
narrow cleavage lines are identical. In one case the surfaces 
have remained so closely appressed as to be indistinguishable, 
while in the other they have been pulled apart at once. 
The two conditions can hardly be regarded as distinct stages 
