The encysted oncosphere develops a delicate monocellular inner germinal layer which secretes a PAS-positive polysaccharide protein complex, the acellular laminated layer that forms around the germinal layer (Fig. 3.9).. This endocyst grows up to 2 mm in thickness. Even though the laminated layer is delicate, it protects the inner more fragile germinal layer. The endocyst is the true wall of the cyst. The germinal layer produces clear fluid which attains a pressure of up to 300 mm of water, keeping the endocyst in intimate contact with the pericyst. The endocyst receives its sustenance from the pericyst.

Although hydatid fluid has been studied extensively, the mechanisms by which fluid and other substances are transferred into the avascular cyst are not well understood. It is certain, however, that transfer does take place since host proteins and some lipids that are not synthesized by Echinococcus are found in the cyst fluid. Diffusion, endocytosis, specific filter or transport mechanisms, and fissures in cyst membranes have been suggested to explain transport, although evidence for fissures is not convincing.

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