Ans. C. Gastrulation.(Ref: Langman's Medical Embryology 9th/pg. 65).Longman's Medical Embryology 9th/pg. 65.........."The most characteristic event occurring during the third week of gestation is gastrulation, the process that establishes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo. Gastrulation begins with formation of the primitive streak on the surface of the epiblast. Thus, the epiblast, through the process of gastrulation, is the source of all of the germ layers, and cells in these layers will give rise to all of the tissues and organs in the embryo".GASTRULATION# Definition: Gastrulation is a process that establishes the three primary germ layers: ectoderm, mesoderm, and endoderm.- This process is first indicated by the formation of the primitive streak within the epiblast.- The formation of the primitive streak is one of the first signs of gastrulation.# It is characterized as a furrow in the midline of the embryonic disk at the future caudal end of the embryo. This furrow is formed by the ingression of epiblast cells that will go on to form the definitive endoderm and mesoderm by replacing hypoblast cells.# The epiblast cells then develop into the definitive ectoderm. Gastrulation, or formation of the germ layers, continues in caudal segments while cranial structures are differentiating, causing embryo to develop cephalocaudally.Educational points:# Week 2: Bilaminar germ disc# Week 3: Trilaminar germ disc (Gastrulation, Primitive streak, notochord, mesoderm and its organization, and neural plate begin to form)# Week 4: Four limbs appear.Heart begins to beat.# Week 3-8 (Embryonic Period): Neural tube formed by neuroectoderm,closes by wk 4.0rganogenesis. Extremely susceptible to teratogens.# Week 6: Fetal cardiac activity visible by transvaginal ultrasound.# Week 10: Genitalia have male/female characteristics.Tumors Associated With GastrulationSometimes, remnants of the primitive streak persist in the sacrococcygeal region. These clusters of pluripotent cells proliferate and form tumors, known as sacrococcygeal teratomas, that commonly contain tissues derived from all three germ layers. This is the most common tumor in newborns. These tumors may also arise from primordial germ cells (PGCs) that fail to migrate to the gonadal ridge.