Each section was then laid flat inside the lid of an
inverted BEEM capsule (Better Equipment for Electron
Microscopy, Bronx, N. Y.) from which the tip had been
removed with a razor blade. The capsules were filled with
Epon and polymerized at 60~ for 2 days.
Sections of 600-800 A thickness were cut with diamond
knives on a Porter-Blum MT2-B microtome (Ivan
Sorvall, Inc., Norwalk, Conn.). Except for unincubated
controls which were handled in the same manner as tissue
for routine microscopy, sections were always cut as close
to the surface of the slice as possible. As sectioning
proceeded from the surface to the interior, preservation
of altrastructure improved but the incidence of reaction
product decreased. Each ribbon was viewed in the
electron microscope (Siemens Elmiskop 101) and photographs
were taken in a region having both acceptable
preservation of ultrastructure and specific reaction product.
Sections were always photographed before staining
with lead citrate so that very small deposits of reaction
product could be detected (31). This also prevented the
spurious identification of lead stain precipitate as reaction
product. After this initial photography, the sections
were stained for 5 min in lead citrate (43) to improve
contrast, and photographed again.