Eleven embryos, ranging from partly to fully bilaminar blastocysts, were obtained from the bandicoots I. macrourus and P. nasuta and were examined by light and electron microscopy. The morphological changes which occurred during the differentiation of the endoderm and ectoderm are described.
The shell membrane was thinner than it was in unilaminar blastocysts and had a deposit of material of irregular thickness on its outer surface. The mucoid coat and zona pellucida were absent or discontinuous.
Endoderm formation was first observed in blastocysts about 1.0 mm in diameter. Cells migrated inwards from regions of thickened protoderm to form a continuous layer of similar flattened cells - the endoderm - beneath the protoderm, which then became the ectoderm. The blastocysts were fully bilaminar when they were 1.5-1.9 mm in diameter. At this stage the ectoderm was composed of two distinct regions, an embryonic region of cuboidal cells and a non-embryonic region of flattened cells resembling the cells of the endoderm.
The formation of the endoderm in bandicoots closely resembles that described in other marsupials, except the American opossum Didelphis virginiana in which endoderm cells are released into the blastocoele before they form a continuous layer.