Observations with the electron microscope have shown that 4 major layers can be distinguished in the cyst wall:

(a) an outer tanned-protein layer, consisting of a meshwork of irregular bodies made up of cigar-shaped particles;

(b) a predominantly mucopolysaccharide, finely-fibrous layer, closely adherent to the tanned layer;

(c) an inner, mainly mucopolysaccharide layer, which can be resolved into two layers differing in fine texture;

(d) a dense, compact layer, composed of numerous protein sheets stabilized by disulphide linkages. This layer is formed from tightly wound scrolls, developed in intracellular vacuoles, which are unrolled at the surface of the animal after secretion.

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