22- Drawing by Ernest Haeckel illustrates general laws of embryology proposed by Karl Earnest von Baer.
Studies comparing early vertebrate embryology found that different species have many features in common.
- There is a common pattern to all vertebrate development:
- Ectoderm forms skin and nervous system.
- Mesoderm forms connective tissue, blood cells, heart, and muscle.
- Endoderm forms the lining of respiratory and digestive tubes.
- General features appear before specialized features. After gastrulation many vertebrate embryos have similar features. For example, they all have notochords and spinal cords.
- Specialized features develop from general features. Many vertebrates develop limb buds, but different specialized features, such as wings or hands, appear later.