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THE POPULAR SCIENCE MONTHLY 



Arcella vulgaris, a common fresh-water shelled rhizopod, has a much 

 more simple life-history. The nucleus of the young form soon divides, 



so that most Arcella specimens con- 

 tain two primary nuclei (Fig. 9). 

 From the very outset, furthermore, 

 each nucleus secretes a chromatin 

 substance which collects in a zone 

 about the periphery of the cell. This 

 substance is not granular like the 

 chromidia, but has a similar origin 

 from the nucleus, and has the same 

 germ-plasmic fate as chromidia, so 

 that Hertwig was justified in calling 

 it a " chromidial net." When the 

 organism is mature, minute nuclei 

 condense out of the substance of this 

 network, hundreds of them being 

 formed (Fig. 10). As in Poly- 

 stomella, each nucleus becomes surrounded by a zone of protoplasm, and, 

 finally, a large number of small swarmers emerge from the shell mouth, 

 leaving behind in the shell the two primary nuclei and a portion of the 

 protoplasm as a degenerating residue. The swarmers are dimorphic, 



Fig. 9. 



Original. 





Fig. 10. 



After Hertwig. 



some are macrogametes, some microgametes (Fig. 11). These fuse 

 two by two, a macrogamete with a microgamete, and the resulting 

 zygote develops into the normal form. 



