Best Answer: The endoderm is one of the germ layers formed during animal embryogenesis. It forms the epithelial lining of multiple systems, such as the gastrointestinal tract. The mesoderm germ layer forms in the embryos of animals more complex than cnidarians, making them triploblastic. During gastrulation, some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and the ectoderm, which contributes to the circulatory system. This layer also develops into a coelom within more complex organisms. The ectoderm is the start of a tissue that covers the body surfaces. It emerges first and forms from the outermost of the germ layers. In chickens, this would consist of feathers.