Final answer:
Gastrulation in chick embryos begins 24 hours after fertilization with the formation of the primitive streak, which leads to a three-layered disc of the endoderm, mesoderm, and ectoderm. The following stages involve regression of the primitive streak and the start of neurulation, setting the stage for organogenesis and the formation of the central nervous system.
Step-by-step explanation:
Chick Gastrulation and Embryonic Development
The process of gastrulation in chick embryos begins approximately 24 hours after fertilization with the full extension of the primitive streak. This marks a transformation from a two-layered to a three-layered cell disc, producing the three germ layers known as the endoderm, mesoderm, and ectoderm.
At the 25-hour mark, known as the two somite stage, and then progressing to the four somite stage at around 27 hours, gastrulation continues as cells migrate through the primitive streak. By the 28th hour, the primitive streak has regressed to the caudal region, signaling the initiation of neurulation. During neurulation, the ectoderm forms the neural plate, which will fold inwards to create the neural tube, the precursor to the central nervous system.
The notochord, derived from mesoderm, acts as the structural basis for the future intervertebral discs and defines the embryonic axis. Once the primitive streak regresses, it leaves the notochord in its wake. Semites on either side of the neural tube will develop into the axial skeleton, skeletal muscle, and dermis. Organogenesis will follow, laying the groundwork for development of all organ systems.