Final answer:
Animal development starts with fertilization, forming a zygote. This zygote undergoes cleavage to become a blastula, which then undergoes gastrulation to form a gastrula with three germ layers: ectoderm, mesoderm, and endoderm, each giving rise to different tissues and organ systems.
Step-by-step explanation:
Stages of Animal Development from Fertilization to Gastrula
The development of an animal from fertilization to the gastrula stage constitutes a complex and finely regulated series of events. First, fertilization occurs, where a sperm and an egg fuse, creating a zygote. This zygote then undergoes a series of mitotic cell divisions known as cleavage. The result of cleavage is a structurally simple yet critical stage called the blastula, often a hollow ball of cells. Following this, through a process known as gastrulation, the blastula restructures into a gastrula, a form that contains three primary germ layers - the ectoderm, mesoderm, and endoderm.
During gastrulation, cells from the surface of the blastula move inward to form the archenteron, which is the primitive gut, and the opening of the archenteron, known as the blastopore. The ectoderm ultimately gives rise to body coverings and the nervous system, while the mesoderm develops into many of the body's muscles and connective tissues. Meanwhile, the endoderm forms the lining of the digestive tract and associated organs. These germ layers lay the foundational organization from which the embryo will develop complex tissues and organ systems during subsequent stages of development.
Embryonic development progresses through multiple stages: starting from the zygote formation, to cleavage leading to the blastula, followed by gastrulation producing the gastrula with its three germ layers, and continuing towards organ formation and fetal development. The precise mechanisms and genetic controls governing these processes are an active area of biological research, highlighting the intricacy and beauty of animal development.