possible to identify subpopulations of the scrapie agent.6 This observa-

tion has provided a better understanding of how these unusual neuro-

pathic agents adapt to different species of animals, the principal mecha-

nism being selection of subpopulations. It has also revealed a diversity in

the pathologic response to infection as influenced by the agent-host

combination.7

Ag 1A—Spongiform degeneration of cerebral cortex in experimental kuru. (H&E, x 400).

B—Astrocytic hypertrophy in the cerebral cortex in Creutzfeldt-Jakob disease. (Cajal’s

gold sublimate, x 200). C—Dilated nerve cell process containing curled membrane frag-

ments in experimental kuru. (Line = 1 sm). Photographs from Marsh9 and published

with permission of Academic Press.