Primordial germ cells in normal and transected duck blastoderms.

Suggestive evidence for the extragonadal origin of germ cells in birds was first presented by Swift (1914), who described primordial germ cells in the chick embryo at as early a stage as the primitive streak. According to Swift, primordial germ cells are originally located extra-embryonically in the anterior part of the blastoderm and occupy a crescent-shaped region ('germinal crescent') on the boundary between area opaca and area pellucida. Swift also found that primordial germ cells later enter into the blood vessels, circulate together with the blood throughout the whole blastoderm and finally penetrate into the genital ridges, where they become definitive germ cells. Swift's views have been confirmed in numerous descriptive and experimental investigations. Among the latter, the publications of Willier (1937), Simon (1960) and Dubois (1964a,b, 1965a, 6, 1966) merit special attention. Dubois finally proved that the genital ridges exert a strong chemotactic influence on the primordial germ cells. Duck embryos have not as yet been so intensively investigated in this respect as those of chicks. The recently published reports by Fargeix (1966, \961a,b,c) and Fargeix & Theilleux (1966) suggest that the history of primordial germ cells in the duck is similar to that in the chick. Although the history of primordial germ cells (PCGs) in birds has been thoroughly investigated, the cause of their differentiation at a given time and in a given territory is still unknown. One of the possible ways of attacking this problem is to disturb the original organization of the blastoderm by transection, which results in the formation of twin embryos. Studies on the transection in ovo of bird blastoderms have been made for several years by Lutz and his co-workers (Lutz, 1949, 1953, 1955, 1964, 1965; Fargeix, 1964a,b, 1965, I961a,b,c) and have shown that in the duck, as well as in the quail, goose and turkey, the embryo can form from any sufficiently large fragment of the blastoderm. The method used by these authors (Wolff & Lutz, 1947; Lutz, 1949) makes it possible to obtain twin embryos, which may survive as long as 10 days of incubation.