o (^"^ y 



Fig. 4. 



Electron micrograph of flagellar base and rootlet, r, 

 rootlet; fl, flagellar fibrils; m, mitochondrion. 



Fig. 5. 



Cross section through the mitochondrion and basal body 

 of a zoospore, bb, basal body; r, rootlet. 



as to be scarcely visible in the bottom of the 

 Spinco tube. The washed cap ribosomes con- 

 tain about 63% RNA and 27% protein. 



Before launching into some of the experi- 

 mental work on the formation of the spores, I 

 would like to discuss very briefly some of our 

 ideas concerning the structure and function of 

 the nuclear cap. The observation that all the 

 cellular ribosomes are packaged in this peculiar 

 structure surrounded by a membrane raised 

 some rather obvious questions as to its function. 

 First, where do the cap ribosomes come from? 

 One can guess, and I think our original guess, 

 that they come from the cytoplasm, turned out 

 to be correct, although it obviously had to be 

 proved. It is reasonable to expect that the spores 

 might conserve their ribosomes. However, they 

 might also be made essentially in situ, at the 

 time the cap is formed, by degradation of 

 pre-existing ribosomes followed by resynthesis 

 in a new location. 



A second, and perhaps even more interest- 

 ing, problem concerns the function of the cap 

 for the spore. Blastocladiella is not the only 

 fungus to produce these; they are produced by 

 a whole series of fungi. But why on earth do 

 they form such unusual structures? First, it 

 may be that the cap serves as a storage 



Fig. 6. 

 Isolated zoospore nuclear caps. 



reservoir of RNA and protein for early germi- 

 nation. It is possible that the cell degrades the 

 cap ribonucleoprotein and uses the products to 

 make new ribosomes to start growth. Alterna- 

 tively, it might store the ribosomes during the 

 non- synthetic zoospore stage. I should em- 

 phasize that the zoospore is motile and me- 

 tabolically active, but it doesn't grow. The 



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