The present papar described a course of formation of the dark spot, which comprised, a mass of the melanophores and which was induced by the encysted larva, on the skin in Oryzias latipes. Developmental observations were carried out over a period from October to November.
Direct observation of a parasitized area showed that, in this fish, the pigment cells were newly formed in the area where the parasite infected, but not gathered there by migration of the melanophores which had existed in the adjacent region.
The process of differentiation of the newly developed pigment cells paralleled with that of the normal melanophores.
The development of the pigment cells around the encysted parasite did not proceed after the death of the larval parasite.