A description is given of cells termed lipo-protein cells in the blood of Carcinus maenas, and of their development from amoebocytes. These cells exhibit a marked cycle of appearance and disappearance correlated with the moult. They appear towards the end of the instar, reach their maximum development immediately before the epicuticle of the new cuticle is secreted, and disappear soon after the moult. They show positive reactions for the presence of lipids, aromatic substances, tyrosine, and peptide linkages. As the epicuticle and pre-exuvial endocuticle are secreted, the colouring with Sudan black B and the Millon reaction in the cells decrease in intensity. The cells are believed to synthesize lipo-protein or extract it from the haemolymph, and to transport it to the epidermis and thence to the developing cuticle, thus contributing to the secretion of the epicuticle and impregnating the pre-exuvial endocuticle. The lipo-protein cells are compared with the oenocytes of insects and with the granulocytes and vacuolar cells or leberidocytes of spiders.

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