The fine structure of bivalents from golden hamster and house cricket spermatocytes has been studied with a whole mount surface-spreading method combined with negative staining. The elements of the synaptonemal complex show detail of structure which is absent in other preparative procedures. The transverse filaments found in the central region of the synaptonemal complex from both species are straight and have a similar width, 1 6–1 8 nm These filaments occur mainly in bundles The central element differs in architecture in the two species In hamster bivalents it is formed of longitudinal stretches of filaments 1.6–1 8 nm wide and a small amount of an amorphous material similar to that of the lateral elements In the cricket, the central element contains transverse fibrils which are continuous with the transverse filaments of the central region, and an amorphous material lying mainly along the sides of the central element All of the components of the central region of the synaptonemal complex are resistant to pancreatic DNase. The overlapping ends of the transverse filaments, together with additional protein material, make up the central element The widespread occurrence and close morphological and histochemical interspecies similarities of the transverse filaments indicate that they serve an essential role, probably one concerned with holding synapsed bivalents together via the lateral elements. Restrictions placed by the observations reported here on current models of the synaptonemal complex are discussed.

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