Final answer:
The development of scales and feathers involves both the initial formation of a placode and the overlapping expression domains of Bmp2 and Shh.
Step-by-step explanation:
The development of both scales and feathers involves the initial formation of a placode, which is an area of thickened ectoderm that gives rise to these skin appendages. Furthermore, studies have shown that there are overlapping expression domains of Bmp2 and Shh (Bone Morphogenetic Protein 2 and Sonic Hedgehog), which are crucial signaling molecules for the development of these structures.
Therefore, both a) the initial formation of a placode and b) overlapping expression domains of Bmp2 and Shh are correct in describing the similarities in the development pathways of scales and feathers. These processes are regulated by growth factors and signaling molecules called growth factors. For example, during the formation of the neural system, growth factors signal some cells at the edge of the ectoderm to become epidermal cells, while the remaining cells in the center form the neural plate.