Observations on the Fine Structure of the Schizonts of Haemoproteus columbae Kruse*

SYNOPSIS. The schizonts of Haemoproteus columbae resemble the exoerythrocytic schizonts of avian Plasmodium in their fine structure. Haemoproteus infects endothelial cells and grows several hundredfold in volume, destroying the cytoplasm and nucleus of the host cell. The schizont's plasma membrane is trilamellar with a dense outer lamella. Some schizonts have micropores in their plasma membranes, but there is no evidence for ingestion thru them. Instead, numerous vesicles and channels fill the host cell cytoplasm and give its plasma membrane and periparasitic vacuolar membrane the appearance of active pinocytosis. The parasite's membrane shows no sign of pinocytosis, indicating that it probably feeds by diffusion. The growing schizont has numerous mitochondria, nuclei, and ribosome‐rich cytoplasm which contains electron‐lucent vacuoles and clefts. The latter appear to be artifacts of fixation.