原线（primitive streak）又可称为原条，是许多动物早期胚胎具有的构造，存在于鸟类、哺乳类、爬虫类胚胎发育的过程中。

原线在许多重要的胚胎发育时期扮演中线的角色，包括胚胎的两侧对称、胚胎原肠形成的位置以及各胚层分布方式等。往后的细胞重排(包含原肠形成、三胚层形成)也根据这条中线进行。原肠形成是第一个破坏胚胎对称的发展阶段，也使得胚胎具有前后之别。同时细胞移动也同时进行着，上下胚层逐渐到达三胚层的原始位置，开始分化形成特殊功能的成体细胞。

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.