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.

1. 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.
2. General features appear before specialized features. After gastrulation many vertebrate embryos have similar features. For example, they all have notochords and spinal cords.
3. Specialized features develop from general features. Many vertebrates develop limb buds, but different specialized features, such as wings or hands, appear later.