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Figs. 16-18. Electron micrographs of spermatozoa of the sea urchin, 

 Strongylocentrotus pulcherrimus. (From Dan, Biol. Bull., 1952.) 16, after 

 agglutination with egg' water. 17, after treatment with alkaline sea water. 

 18, in sea water (control). 



Figs. 19-20. Electron micrographs of spermatozoa of Echinocardium 

 cordatum. (From Rothschild and Tyler, Exptl. Cell Research, 1955.) 



