Image Caption : 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.
The primitive streak is a structure that forms in the blastula during the early stages of avian, reptilian and mammalian embryonic development. It forms on the dorsal (back) face of the developing embryo, toward the caudal or posterior end.
The presence of the primitive streak will establish bilateral symmetry, determine the site of gastrulation and initiate germ layer formation. To form the streak, reptiles, birds and mammals arrange mesenchymal cells along the prospective midline, establishing the second embryonic axis, as well as the place where cells will ingress and migrate during the process of gastrulation and germ layer formation. The primitive streak extends through this midline and creates the left–right and cranial–caudal body axes, and marks the beginning of gastrulation. This process involves the ingression of mesoderm progenitors and their migration to their ultimate position, where they will differentiate into the mesoderm germ layer that, together with endoderm and ectoderm germ layers, will give rise to all the tissues of the adult organism.
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