Fig. 2.

Fig. 2. Schematic diagram of feather tract and types of natal down formation in zebra finch and chicken. Zebra finch embryos show two types of feather formation: Type I feather formation (open circles), in which the feather buds do not develop into downy feather; Type II feather formation (black circles), in which the feather buds develop into downy feathers, which are later replaced by contour feathers. Chicken embryos exhibit only the Type II feather formation. E8, E9, and E12: embryo day 8, 9 and 12, respectively. D7: 7 days post-hatch. Scale bar = 0.1 cm. The figure was modified from our previous study (Chen et al. 2017).