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parts of the "acrosome" in spermatozoa of several species of bi- 

 valve molluscs (Figs. 1, 4, 7, 11): a distal section containing 

 what these authors believe to be an egg-membrane lysin, an axial 

 structure possibly related to the precursor of a filament, and a 

 basal structure which may be concerned with the extrusion of the 

 filament. The axial structure described in these acrosomal re- 

 gions would seem to resemble the "stereocil" reported in the 

 spermatozoa of some prosobranch molluscs and in Lumbricus 

 (Grasse and Tuzet, 1933; Tuzet, 1950). 



Fig. 14. Profiles of heads of sea urchin spermatozoa. (From Afzelius, 

 Z. Zellforsch., 1955.) a-b, Strongijlocentrotus droebachiensis; c, Psammechi- 

 nus miliaris; d, Echinus esculentus; e, Echinocardhim cordatum. 



Afzelius ( 1955 ) has made electron microscope studies of thin 

 sections of spermatozoa of four species of sea urchin. They reveal 

 that a rather dense acrosomal particle lies in the tip of the acro- 

 somal region. Beneath this particle the nucleus shows a depres- 

 sion or "cave" filled with material less osmiophilic than that of 

 the particle (Figs. 14—15). In one species, Echinocardium corda- 

 tum, the acrosomal region is elongated in a manner unusual for 

 sea urchins (Figs. 14e and 15B). Nevertheless, the acrosomal 

 particle occupies the apical position. Beneath the particle a long 

 stalk is apparently filled with material like that of the "cave" in 



