Archive image from page 101 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 82 THE DEVELOPMENT OF THE CHICK entoderm for a considerable period after the formation of the head- fold. A longitudinal section shows the head-process as an append- age to the anterior end of the primitive streak, or the primitive knot (Fig. 42). Fig. 43. — Diagrams to illustrate the theory of concrescence as applied to the primitive streak of the bird. The central area bounded by the broken line represents the pellucid area; ex
Archive image from page 101 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 82 THE DEVELOPMENT OF THE CHICK entoderm for a considerable period after the formation of the head- fold. A longitudinal section shows the head-process as an append- age to the anterior end of the primitive streak, or the primitive knot (Fig. 42). Fig. 43. — Diagrams to illustrate the theory of concrescence as applied to the primitive streak of the bird. The central area bounded by the broken line represents the pellucid area; external to this is the area opaca, showing as concentric zones the germ-wall (G. W.), the zone of junction (Z. J.), and the margin of overgrowth (M. O.). m. n., Marginal notch. For de- scription see text. The most obvious interpretation of the head-process is as an outgrowth from the primitive knot. But another, and more probable interpretation in view of all the facts, is that the head- process is a later stage of the anterior end of the primitive streak;
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