A Human Embryo of Streeter Age Group XII

Abstract

A human embryo that is thought to belong to age group XII of Streeter's developmental horizon, is described. This embryo was obtained from a tubal gestation to a 35-year old woman. Although the large part of head portion was lost 212 serial sections could be made in fairly good condition. The ovulation age of the embryo was 28 days. The length of the embryo and the number of somites could not be calculated because of the lost portion. This embryo was characterized by forelimb bud, optic vesicle, lung bud, bilateral active invasion of epithelial cords in hepatic diverticulum, narrow yolk sac, allantois, prominent mesonephric duct and meso" nephric tubules, single atrium and urogenital sinus