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 



FIG. 15. Cell-cleavage in sporangium of Pilobolut 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, sporangia! 



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 situ. 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, 



