CELL-DIVISION. 39 
at a very late period strands of protoplasm are seen connecting 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 focussing up and down, to lie connected. There can be 
little doubt, however, that a considerable part of cleavage of the columella is 
accomplished by flattening and lateral fusion of originally ellipsoidal or spheri- 
cal vacuoles ; that is, the cleavage is not entirely by a furrow from the plasma 
Fic. 15,—Cell-cleavage in sporangium of Pilobolus crystallinus.—(After Harper.) 
A, median section at stage when columella is forming. 
B, section of spore-plasma from base of sporangium, showing surface cleavage-furrows ; a, sporangial 
wall. 
C, section of portion of upper part of a sporangium, showing irregular sausage-shaped bodies formed by 
cleavage of spore-plasma. 
D, similar to C, but older, showing uninucleate masses (protospores). 
membrane at the mouth of the sporangiophore, but is at least in part a process 
of separation by excretion of a liquid into vacuoles and their fusion side by side 
in sifu. These vacuoles are not situated on the extreme boundary of the pro- 
toplasm adjacent to the large central vacuole, but placed where the dense spore- 
plasma first becomes characteristically spongy. At the base of the sporangium 
indeed, they cut through plasma as dense as the densest spore-plasma of the 
sporangium. Why the cell-wall of the columella could not be deposited on the 
surface of the central vacuole, as well as on the surface of the small vacuoles, 
