100 AQUATIC PHYCOMYCETES 



more nuclei. Several large nuclei appear in the sporangial rudiment 

 as it expands but it is not known whether they have been derived from 

 the first nucleus or have migrated there. As the rudiment continues 

 to enlarge and become more globose, it contains more nuclei and in 

 the center there is an aggregation of deeply staining granules. When 

 the sporangium is mature, the filament bearing it is usually devoid of 

 nuclei save for the swollen subsporangial region which is nucleated. 

 In intercalary sporangia, concomitant vegetative elements on either 

 side are similarly nucleated. 



As maturation of the sporangium continues, the central, deeply 

 staining mass disintegrates and becomes a densely granular reticulum 

 which spreads throughout the swelling. The sporangial nuclei are align- 

 ed on the strands of this reticulum, some elements of which apparently 

 assist in the formation of the basal cross wall. At this time, too, the 

 sporangial wall thickens and the discharge papilla begins to form. 

 The granules of the reticulum disperse and the cytoplasm becomes 

 uniformly granular, except just beneath the papilla, a region which 

 seems devoid of stainable cytoplasm. The peripheral sporangial wall 

 thickens and the arched operculum appears as a lightly stained dome, 

 thicker in the center and tapering off toward the edges. Where the oper- 

 culum is attached to the rest of the sporangium the wall does not 

 thicken but remains as a thin ring, obviously the region of dehiscence. 

 By the time the operculum is fully developed, the basal cross wall 

 has thickened to the same degree as the wall of the sporangium. 



The cytoplasm now begins to be furrowed by elongate vacuoles 

 which extend centripetally from the periphery. Other vacuoles progress 

 laterally until each nucleus is surrounded by a mass of cytoplasm. As 

 deep-staining material collects around each nucleus to form a large 

 nuclear cap, the remainder of the cytoplasm in the cleaved-out 

 masses becomes clearer. These masses then swell so as to obscure the 

 spore origins and the nuclear caps become more compact. The nucleolus 

 is visible in the center of the nucleus before as well as after aggregation 

 of the granules making up the cap, evidence, Roberts believes, that it 

 is not concerned with nuclear-cap formation. His observations support 

 Karling's (1937b) contention that the nuclear cap is extranuclear in 

 origin and may represent food material utilized during spore germi- 

 nation. 



