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Histological studies, carbon, carmine and vital dye marking of in ovo and explanted blastoderms have provided evidence for the following: (1) The cellular Frecursors of all three primary germ layers in the unincubated blastoderm are topographically represented by a disc-shaped, epithelial-like and complete uppermost layer; and ring-shaped, incomplete and congruent middle and lowest layers below the uppermost layer. ( 2 ) There is a gradient in cell population density decreasing from posterior to anterior. (3) Centripetal extension of a sheet of lowest and adjacent middle layer cells, primarily from the posterior portion of the ring but also from all points on its circumference, results in closing of the ring and completion of the middle and lowest layers as coherent discs. (4) No movement (imagination or involution) of uppermost, surface a w a s as coherent sheets into either the middle or lowest layers could be demonstrated in any region, including the primitive streak. (5) Embryonic germ layers arise in situ by proliferation from the streak. ( 6 ) The streak in its morphology and function is more like a blastema than a blastopore. ( 7 ) As an elongated growth center, the streak has its origin from the marginal zone ring, itself a circular blastema with multiple streak-and embryo body-forming capacities.