Early mammalian embryonic development.

This report examines the major events occurring during the first month of development in the human embryo. Developmental events are detailed, beginning with cleavage of the zygote and ending with the formation of the three primary germ layers. Certain aspects of early human development are compared with that in lower forms of vertebrates to emphasize species differences in these basic embryological processes. A discussion of the formation of identical twins is included, since it has its inception during the first month of development.