The origin of the primary endoderm was studied by combining experimentally eight-cell embryos and ICMs isolated by microsurgery from blastocysts homozygous for different alleles at the glucosephosphate isomerase (GPI-1) locus. The results confirm that the primary endoderm cells are derived from the ICM. The yolk sac splanchnopleure, which is composed of both extraembryonic endoderm and mesoderm, usually displayed the ICM GPI isoenzyme genotype. It must be concluded that the embryonic endoderm (visceral endoderm) and the parietal endoderm have a common origin and develop from the primitive ectoderm of the egg cylinder.

Department of Zoology Faculty of Science Alexandria University Egypt