C. B. METZ 57 



agent is weakly bound to the sperm surface since, on standing, 

 sperm suspensions gradually charge the sea water with mem- 

 brane lysin (Tyler, 1939b; Krauss, 1950a). The lysin can be ex- 

 tracted with alkali (Krauss, 1950a) or by freeze thawing (Tyler, 

 1939b). The lytic agent and the Megathura antifertilizin also 

 present in frozen-thawed extracts are distinct since the two agents 

 have different heat stabilities [the lytic agent is rapidly destroyed 

 at 60° C; antifertilizin remains active for some time at 100° C 

 (Tyler, 1939b; Krauss, 1950a)], and they can be extracted sep- 

 arately [the lysin with alkali; antifertilizin with acid (Krauss, 

 1950a)]. The Megathura egg membrane lysin is evidently a labile 

 protein (Tyler, 1939b; Krauss, 1950a). Release of the lysin by 

 alkali is associated with dissolution of the sperm acrosome ( Tyler, 

 1949). Krauss (1950a) suggests that the lysin may act by reduc- 

 ing disulfide linkages since cysteine and other sulfhydryl reagents 

 dissolved the membrane. However, he was unable to demonstrate 

 free SH groups in the native extract or block the action of the 

 lysin with iodoacetate. 



Unfertilized Mytilus edulis eggs are surrounded by a thin, tight 

 vitelline membrane. This membrane is lysed by frozen-thawed 

 or acid extracts of Mytilus sperm (Berg, 1949, 1950). Further- 

 more, if eggs are permitted to cleave in these extracts, the result- 

 ing blastomeres are very loosely joined. The intercellular cement 

 is evidently attacked by the extract. The two effects may result 

 from separate agents, for the membrane lysin is more readily de- 

 stroyed by heat than the cement lysin. Furthermore, only the 

 latter action is inhibited in concentrated sperm extracts. Both 

 agents appear to be proteins (Berg, 1950). Lytic agents com- 

 parable to those from Megathura and Mytilus sperm have been 

 demonstrated in sperm extracts of several other molluscs (Tyler, 

 1939b; von Medem, 1942, 1945) and the amphibian, Discoglossus 

 pictus (Hibbard, 1928). Concentrated sperm suspensions have 

 similar action on the egg membrane of the annelid Pomatoceros 

 triquester (Monroy, 1948). In fact, it is reasonable to assume 

 some such agent for species with eggs that are surrounded by 

 heavy membranes, even when the lysin is not found in sperm ex- 

 tracts [e.g., Cerebratulus (Metz, unpublished)]. 



