Observations on Ichthyosporidium giganteum (Microsporida) with particular reference to the host-parasite relations during merogony.

Connective tissue cells, particularly fibroblasts, of the fish Leiostomus xanthurus Lacépède respond to the invading microsporidian parasite Ichthyosporidium sp. [assumed to be identical with Ichthyosporidium giganteum (Thélohan)] by proliferating themselves, coalescing into a syncytium, synthesizing copious amounts of cytoplasm around the parasites, and walling off the parasitized islands of cytoplasm with fibrous capsules. The resulting cysts are xenoparasitic complexes of the syncytial xenoma type, clearly different from the cell hypertrophy tumor (xenoma sensu Weissenberg) exemplified by the Glugea cyst. These findings involve a new concept of the structure and host-parasite relations of Ichthyosporidium. Formerly, the parasitized masses of cytoplasm were interpreted as extracellular plasmodial stages of the parasite (stages uncharacteristic of the microsporidia), while the parasites themselves were interpreted as nuclei of the "plasmodia." Actually, the parasite undergoes merogony in parasitophorous vacuoles which coalesce before sporogony begins. The nuclei of the mermonts are very small chromatin granules, becoming transformed into large basophilic diplokarya of the sporonts. Sporulation is diplokaryotic throughout, the diplokarya becoming reduced in size through 2 steps during sporogony.