The animation begins with the dorsal view of the bilaminar germ disc in a 14-day-old embryo. A section is made at the level of the primitive streak, with the cut surface magnified and rotated toward the spectator. The epiblast and the hypoblast are shown along with other structural details.

Formation of the endoderm. This sequence shows the migration of the epiblastic cells toward the primitive streak, their invagination beneath it and the invasion of the hypoblast by the epiblastic cells. In an embryo during the course of the 16th day, the endoderm forms from the epiblastic cells. Simultaneously, the epiblastic cells continue to migrate between the epiblast and the endoderm, forming the intraembryonic mesoderm. The diagram indicates the extraembryonic mesoderm.

Formation of intraembryonic mesoderm. The sequence starts at the beginning of the 16th day, when the first intraembryonic cells appear. The sequence shows the spreading of the intraembryonic mesoderm between the epiblast (ectoderm) and the endoderm in a 17-day-old embryo at the stage of the trilaminar germ disc.

Conjoined equal and unequal twins are shown as malformations (animated in a special chapter dedicated to the development of twins).

(This animation is essential for students of medicine, veterinary medicine, and biology and for departments of anatomy, histology, embryology and cell biology; it is also recommended for students of stomatology as well as for departments of pediatrics.)