The structure, histochemistry, and histogenesis of the blood-cells of Phallusia mammillata (Cuvier) have been investigated.

Primitive lymphocytes give rise, by a series of transition stages, to vanadocytes, pigment cells, phagocytes, and cells with acidic vacuoles. There is evidence that most of these cells must eventually leave the blood-stream. During the development of each cell type, RNA appears to be released from the nucleus, and there is a progressive reduction in nuclear volume.

Ninety-eight per cent of the total number of blood-cells are vanadocytes or their precursors. Each vanadocyte contains a number of globules. Each globule has a central core enclosed by a surface membrane containing compound lipid. In the core, a vanadium compound is associated with H2SO4, protein, and carbohydrate material, which may be the precursor of the tunicin of the test. Part of the carbohydrate material is in a dispersed form and can be hydrolysed with 2N H2SO4. The remainder is in the form of birefringent grains which resist hydrolysis, and appears to be a very insoluble polysaccharide.

Those lymphocytes that are destined to form vanadocytes develop vacuoles in their cytoplasm initially. In these vacuoles, which are ringed with granules that are probably mitochondria, granular material capable of reducing OsO4 and AgNO3 is formed. The vacuoles in each cell then unite to form one large vacuole, and refractile granules appear in the perinuclear cytoplasm. These granules then move out from the nucleus in the cytoplasm peripheral to the vacuole. Each refractile granule is then enclosed in a subspherical compartment in which protein material is formed. As the compartments are transformed into the globules of vanadocytes, acid, PAS-positive material, and a vanadium chromogen with reducing properties appear.

The pigment cells contain melanin and oriented submicroscopic crystallites.

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