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OBSERVATIONS ON THE PERMEABILITY PROPERTIES OF THE EGG MEMBRANES OF THE MARSUPIAL, TRICHOSURUS VULPECULA

Summary.: The permeability properties of the shell membrane, mucoid coat, zona pellucida and vitelline membrane of thirteen Trichosurus vulpecula eggs were investigated by incubating them in a mixture of the animal's own blood plasma and one of the following substances : vital toluidine blue (mol. wt. 306) ; horse radish peroxidase (mol. wt. 40,000) ; ferritin (mol. wt. 460,000). The degree of penetration of these tracers was assessed either with the light microscope or electron microscope or both.

Both toluidine blue and peroxidase penetrated the shell, the mucoid coat and the zona pellucida to enter the vitellus within 5 min. In eggs with fully formed shell membranes and mucoid coats, ferritin particles penetrated the superficial portions of the shell in low concentrations. There were fewer particles in the deeper portions of the shell and, in the mucoid coat, they were extremely rare or absent. In eggs either lacking a shell or with a thin newly formed shell, ferritin infiltrated the zona pellucida. It was concluded that the pore size of the shell was such that some filtration of ferritin was occurring. Because of the penetration of substances with such a large molecular weight as ferritin and peroxidase, it was considered that neither the shell, the mucoid coat nor the zona pellucida was significantly involved in regulating the passage of embryonic nutrients or wastes.