The fine structure of the exoerythrocytic stages of Plasmodium gallinaceum was studied in chick-embryo liver and was compared with that of the exoerythrocytic stages of avian malaria parasites in a tissue-culture system. The structure of the parasite in this present study seems to be essentially similar to that grown in vitro. These observations appear to confirm the usefulness of the tissue-culture system for studies of the development of the exoerythrocytic stages of avian malaria parasites, at least from the standpoint of fine structure.
The exoerythrocytic stages of P. gallinaceum in chick-embryo liver are found within the endothelial cells of the sinusoids, an observation coinciding with the findings obtained by light microscopy. As the parasite grows, the endothelial cell is displaced by the large parasite and protrudes into the sinusoidal space. At the end of merozoite formation, the endothelial cell forms a thin shell around a large cyst containing several merozoites. The possibility that the membrane surrounding the parasite in the endothelial cell may be derived from the pinocytotic vesicles or from the endoplasmic reticulum of the host cell was discussed. Occasionally degenerating parasites were observed within a digestive vacuole of the Kupffer's cell. The hepatic cells of chick-embryo liver in general seem not to be affected by the infection with the exoerythrocytic stages of P. gallinaceum, except by the increased number of lipid droplets in the cytoplasm.