AbstractHistological studies, carbon, carmine and vital dye marking ofin ovoand explanted blastoderms have provided evidence for the following: (1) The cellular precursors 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 (invagination or involution) of uppermost, surfaceareasas coherent sheets into either the middle or lowest layers could be demonstrated in any region, including the primitive streak. (5) Embryonic germ layers arisein situby 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 c