PATHOLOGY

Detailed explanation-1: -The beginning of gastrulation is marked by the appearance of a groove in the caudal end of the epiblast layer known as the primitive streak . [6] Thus, the formation of primitive steak firmly establishes the cranial/caudal axis. The primitive streak initially forms via a thickening of cells near the connecting stalk.

Detailed explanation-2: -Gastrulation. Late in the second week after fertilization, gastrulation occurs when a blastula, made up of one layer, folds inward and enlarges to create a gastrula.

Detailed explanation-3: -The first stage in the development of a human being, the embryonic stage, begins with fertilization. Fertilization generally occurs in the first third of the fallopian tube, the canal that connects each ovary to the uterus.