Final answer:
The germ line cells are set aside during gastrulation, a key process in embryonic development that forms the three primary germ layers, which then differentiate into various organs and tissues of the body.
Step-by-step explanation:
The germ line cell is set aside very early during embryonic development, specifically during a process known as gastrulation. During gastrulation, which generally occurs in the second week after fertilization, the embryonic cells differentiate into the three primary germ layers: the endoderm, mesoderm, and ectoderm. The endoderm forms the internal organs and glands, the mesoderm develops into bones, muscles, and the heart, and the ectoderm becomes the skin and nervous system.
The inner cell mass of the blastocyst contains embryonic stem cells, which are pivotal in arranging themselves into these germ layers. Organogenesis follows, characterized by the rapid and precise movements of cells within the embryo, and is where these germ layers develop into the specific tissues and organs of the body through a process called differentiation.
Organs develop from these germ layers through differentiation, during which the embryonic stem cells express specific sets of genes that determine their ultimate cell type. For example, cells in the ectoderm will express genes that cause them to develop into epidermal cells. It is through cellular signaling cascades that this regulation of gene expression and cellular differentiation is achieved.