1. Appearance of the Pharyngeal Arches:
* The most prominent feature of the pharyngula stage is the development of pharyngeal arches, a series of pouches that extend from the pharynx (throat) to the exterior.
* These arches are initially supported by cartilaginous bars and play a crucial role in the development of various structures, including:
* Gill slits: In aquatic chordates, these arches develop into gills for respiration.
* Jaws and ears: In vertebrates, these arches contribute to the development of jaws, hyoid bone, and inner ear structures.
* Thyroid and parathyroid glands: These glands also originate from the pharyngeal arches.
2. Development of the Notochord:
* The notochord is a flexible rod that runs along the dorsal side of the body and provides support.
* During the pharyngula stage, the notochord becomes more prominent and extends the length of the embryo.
3. Neural Tube Formation:
* The neural tube, which will eventually become the brain and spinal cord, begins to form along the dorsal surface of the embryo.
* This tube develops from the neural plate, a thickened area of ectoderm that folds inwards.
4. Somite Formation:
* Somites are blocks of mesoderm (middle germ layer) that develop on either side of the notochord.
* They give rise to muscles, vertebrae, and other structures of the body.
5. Differentiation of Germ Layers:
* By the pharyngula stage, the three germ layers – ectoderm, mesoderm, and endoderm – have differentiated and are actively forming various tissues and organs.
6. Tailbud Formation:
* The tailbud, the posterior end of the embryo, starts to develop, marking the beginning of the tail region.
7. Organogenesis:
* Although the pharyngula stage is primarily characterized by the development of the basic body plan, organogenesis also begins during this period.
* For example, the heart, liver, and kidneys start to form.
8. Limited Mobility:
* The pharyngula stage is a relatively stationary stage, with limited mobility for the embryo.
Overall, the pharyngula stage is a crucial developmental milestone in chordates. It lays the foundation for the formation of many important structures, including the brain, spinal cord, muscles, skeleton, and organs. The development of pharyngeal arches is a key characteristic of this stage and distinguishes it from other developmental stages.