The pigment cell system in the Light Sussex fowl.

L I G H T S U S S E X embryos and day-old chicks have no pigmentation of their plumage. Some pigmentation appears at the bases of the wing and tail primaries during the first week after hatching, and at about 6 weeks each neck feather shows a broad longitudinal medial black stripe. The saddle, flank, and breast feathers remain unpigmented, except in some strains which may show some, black flecks in these feathers as the bird ages. Thus in the adult Light Sussex the flight feathers of the wings and tail show considerable areas of black, the neck feathers have a broad black stripe on each side of the rachis, and the rest of the plumage is white (Plate 1, figs. A, B, C). The question therefore arises as to whether the pigment cells are present and non-functional, or absent, in the white feather of a partly pigmented breed such as the Light Sussex. Hamilton (1940) made a study of the pigment cell systems of wholly white and of coloured breeds of fowl, and of various other species of bird. He showed that in tissue culture the back skin of embryos both of white and of coloured breeds produced melanocytes, but of different kinds. The melanocytes produced in the cultures from coloured breeds appeared to retain their functional activity and to remain expanded for long periods (up to 10 days). He further noticed that in the breeds whose melanocytes showed 'high viability' in culture (coloured breeds), apparently functional melanocytes were found much further distally in the growing feather germs of the adult bird; while in the breeds whose melanocytes showed 'low viability' in culture (white breeds), the few melanocytes found at the bases of the growing feather germs seemed to become 'senile' much more proximally. He thus concluded that the melanocyte of the coloured breed is better able to live and function in the environment found more distally in the feather, therefore lives longer as a functional melanocyte, and so donates more melanin. The melanocyte of the white breed, it is supposed, cannot tolerate the conditions found distally in the feather germ, and so becomes 'senile' relatively quickly before having donated sufficient melanin to give colour to the resulting