Dr. Murai Éva - Gubányi András szerk.: Parasitologia Hungarica 27. (Budapest, 1994)

1992) and Garden (1992) contain a multitude of data on the variations of their development and on the cytologic processes. Relying on recent knowledge obtained on Sphaerospora and Hoferellus species, Lorn and Dyková (1986) discussed in detail the majority of problems addressed also by myself, including e.g. the "enveloped condition", pansporoblast formation, the behaviour of generative cells, the extrasporogonic stages, and intracellular development. Despite the complexity of developmental processes outlined by the above authors, the simplified development models of myxosporeans can be divided into three major types, i.e. Myxobolus-type development, Hoferellus-type development, and Sphaerospora-type development. Myxobolus-type development In this type of development, the form infecting the fish is probably identical with the sporoplasm released from the Actinosporea, which penetrates the epithelium that covers the body surface of the fish, undergoes a limited number of divisions there, and finally travels to the final site of colonization along the nerve tracts or via the blood stream. The infective form starts to develop in a specific cell typical of the given species, and forms a relatively large Plasmodium. Within the Plasmodium, vegetative nuclei and generative cells appear: the latter develop into pansporoblasts in most cases. Many researchers including Lorn and Dyková (1986) conceive pansporoblast formation as the association of two generative cells, one of which surrounds the other and becomes a pericyte, while the surrounded sporocyte forms sporoblasts then a spore. My own observations are more consistent with the view of Shulman (1966) and Uspenskaya (1984), i.e. that the pansporoblast develops through internal cleavage of a single generative cell. As a result, the primary cell gives rise to a pansporoblast while the secondary cell to a sporoblast. If the sporoblast does not divide further, monosporoblastic development shall be spoken of; its division into two parts is designated bisporoblastic development, while its division into several parts is termed polysporoblastic development. The multinucleated spore, which is usually composed of 6 nuclei, develops by division of the sporoblast. The majority of known Myxosporea species also develop in this way. Besides Myxobolus spp., some Thelohanellus, Henneguya and Kudoa spp. also follow that form of development. The further fate of the Plasmodium greatly depends on the type of the attacked cell and its location within the body; therefore, numerous variations may exist within that type of development (Figs 1 and 2). In simpler cases of Myxobolus-lype development the plasmodia develop near the body surface (in the skin, on the fins and gills) and, thus, the spores are released directly into the outside world after the cyst opens up. The majority of species parasitic in organs having efferent ducts (intestine, liver, kidney) leave the fish organism in a similar, rather easy way (Fig. 1). However, from plasmodia developing in the inner organs (spleen, liver and renal parenchyma), heart, brain, muscles and on serous membranes the spores may get into the outworld only via the lymph and blood circulation. In such cases the lymph and blood flow transports the numerous spores released from the disrupted cysts into the intercellular space to the gills, skin, kidney and intestine where they cause capillary obstruction and minor necroses, and from