Notice that the brain and sense organs are fully encased in bone, the skull, the chest cavity is protected by a rhythmic bone structure of ribs, and the abdomenal cavity is largely unprotected by bone.
The bone structure of mammals shows a polarity mediated rhythmically. Mammals completely enclose the brain in a skull cavity made of bone, but leave the organs of the abdominal cavity unprotected by bone, and the rhythmic heart and lungs, intermediate between head and abdomen, partially protected by a rhythmically built rib cage.
In order to generate the tissues and organs of a multicellular organism, different cell types have to be generated during embryonic development. The first step in this process of cellular diversification is the formation of the three germ layers: ectoderm, endoderm and mesoderm. The ectoderm gives rise to the nervous system, epidermis and various neural crest-derived tissues, the endoderm goes on to form the gastrointestinal, respiratory and urinary systems as well as many endocrine glands, and the mesoderm will form the notochord, axial skeleton, cartilage, connective tissue, trunk muscles, kidneys and blood. The endoderm forms the organs of the gastrointestinal and respiratory systems, as well as the thymus, parathyroid, bladder, and urethra.
The embryo can be seen to metamorphose in triadic stages: The main stages of embryonic development are:
- Fertilization: The egg is fertilized by sperm, forming a zygote.
- Cleavage: The zygote undergoes rapid cell division, forming a ball of cells called a morula.
- Blastocyst formation: The morula develops into a blastocyst with an inner cell mass and outer layer.
- Implantation: The blastocyst implants in the uterine wall around day 6-10 after fertilization.
- Gastrulation: Formation of the three germ layers – ectoderm, mesoderm, and endoderm – around week 3.
- Neurulation: Development of the neural tube, which will become the brain and spinal cord, begins around week 3-4.
- Organogenesis: Formation of major organs and body systems occurs from weeks 3-8. Key developments include:
- Week 3-4: Heart begins to beat
- Week 4-5: Limb buds appear
- Week 5: Eyes, ears, and nose start forming
- Week 6-7: Digits form on hands and feet
- Week 7-8: All major organs have begun to form
- Fetal period: After week 8, the embryo is considered a fetus. Organs continue to develop and mature during the fetal period.
Throughout these stages, the embryo grows rapidly in size and complexity as cells divide, migrate, and differentiate into specialized tissues and structures. By the end of the embryonic period at 8 weeks, the embryo has a distinctly human appearance with rudimentary forms of all major organ systems present.
In order to generate the tissues and organs of a multicellular organism, different cell types have to be generated during embryonic development. The first step in this process of cellular diversification is the formation of the three germ layers: ectoderm, endoderm and mesoderm. The ectoderm gives rise to the nervous system, epidermis and various neural crest-derived tissues, the endoderm goes on to form the gastrointestinal, respiratory and urinary systems as well as many endocrine glands, and the mesoderm will form the notochord, axial skeleton, cartilage, connective tissue, trunk muscles, kidneys and blood.