a6o 
STRUCTURE AND LIFE HISTORIES 
a well-marked cleft separates the denser peripheral pro- 
toplasm from the thinner central portion, which now 
develops into a little column, or columella (Fig. 186). In the 
FIG. 186. Rhizopus nigricans. i, Young sporangium, showing cyto- 
plasm and nuclei streaming up the sporangiophore into the sporangium 
and out toward the periphery. 2, Sporangium of nearly full size. The 
differentiation between the dense peripheral and the looser central plasms 
is clearly shown. Nuclei are still passing from the central to the periph- 
eral regions along the fine strands of cytoplasm. X about 200. (After 
D. B. Swingle.) (Cf. Figs. 187 and 188.) 
meantime innumerable furrows develop in the denser plasm, 
first at the periphery and then working their way in from 
the columella cleft. By the time the two sets of furrows 
