Schwarze: Cleavage in Sporangia 



167 



imity (fig. 9, pis. 6-7). This stage is to be compared also to the 

 stage in the formation of the columella in sporangia of .the Zygo- 

 mycetes where the vacuoles flatten and fuse edge to edge. Later, 

 in Vaucheria, a wall is formed between the two membranes and is 

 seen to be convex toward the oogonium (fig. 11, pis. 6-7). The 

 oosphere is now rounded up in the second contraction phase. The 

 protoplasmic mass, which has heretofore conformed to the general 

 outline of the oogonial wall, undergoes contraction until the rather 

 globular or ovoid oosphere is formed. The ripe oosphere contains 

 relatively few chloroplasts, but numerous oil globules, suggesting 

 the chemical condensation processes which have accompanied the 

 extrusion of cell-sap. Such illustrations show clearly that the 

 process of spore formation, whether sexual or asexual, involves 

 rather a marked series of contraction and expansion phases accom- 

 panied by metabolic changes in the protoplasm which result, in 

 general, in the formation of reserve food products, but whose fun- 

 damental chemical nature is at present little known. 



The process of spore formation may be much abbreviated as in 

 Sporodinia grandis, whose spores are short lived and contain little 

 reserve material, or it may be protracted as in Pilobolus crystallinus 

 and Synchytrium decipiens, by the interpolation of an embryonic 

 stage, in which the protospores increase in size, become multi- 

 nucleated, ripen, form a wall, and enter a period of rest before 

 they germinate by a tube in Pilobolus or by zoospore formation in 

 Synchytrium. 



Swingle (50) attributes spore formation in sporangia as due to 

 localized contractions of the protoplasm He does not believe that 

 the nuclei directly influence contraction, but states : " The nuclei 

 determine to some extent just what protoplasm shall constitute 

 each individual spore." 



Recently Harper (25) has suggested that the loss of water is 

 probably least in the vicinity of the nuclei during the shrinking and 

 condensation of the spore plasm, and that this might be a deter- 

 mining factor in the orientation of the cleavage furrows. 



The failure to note the various contraction and expansion phases 

 accompanying the formation of spores in the sporangia of algae 

 and fungi has doubtless led to the erroneous conception of simul- 



