[The pavement lining of the vessels in the earthworm (Lumbricus terrestris)]

Effect of some preparatory methods to make ready earthworm tissues for electron microscopical investigation has been studied in order to reveal structural components of the vascular wall. The method for an immediate fixation of the earthworm tissues in liquid nitrogen with a subsequent additional fixation in cooled 2.5% glutar aldehyde, applied in combination with OsO4-thiocarbohydroside-OsO4 method is optimal. This method of samples preparation gives a reliable information concerning existence of an internal--endothelial--lining in the earthworm vessels. A conclusion is made on a qualitatively new level of cell differentiation of the internal vascular lining in the earthworm, which is called phase of blast transformation of endotheliocytes.