Human Embryo 18 Day Old with Primitive Streak
Computer Generated Image from Micro-MRI, actual size in length = 1.5 mm - This image presents a side-view of an embryo during its third week of development. The age is calculated from the day of fertilization. The embryo is attached to the uterine wall and attains a pear-shaped structure. The white line seen on the embryo is the primitive streak, which establishes the longitudinal axis of the embryo and signals the development of the right and left sides of the body. The primitive streak also indicates where the division of the brain will occur.