Protoplasm of different cells, showing foam structures. A— Pseudopod of a marine 

 rhizopod. The protoplasm only shows foam structure at the point of stimu- 

 lation. B— Epidermic cell of lumbricus. C— Nerve fiber. D— Part of the cell 

 body of a ganglia cell. (A-C after Butschll, D after Held.) 



