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Vertical section of area pellucida and area opaca (left extremity of figure) of blastoderm of a fresh laid egg (unincubated). Labels: S, superficial layer corresponding to epiblast; D, deeper layer, corresponding to hypoblast; and probably in part to mesoblast; M, large "formative cells," filled with yolk granules, and lying on the floor of the segmentation cavity; A, the white yolk immediately underlying the segmentation cavity.

Blastoderm of an Egg

Vertical section of area pellucida and area opaca (left extremity of figure) of blastoderm of a fresh…

"Section of a Hen's Egg before Incubation. a, yolk, showing concentric layers; a', its semi-fluid center; b, inner dense part of the albumen; b', outer thinner part; c, twisted cords of albumen; h, the white spot, or germ cell." -Cooper, 1887

Egg Parts

"Section of a Hen's Egg before Incubation. a, yolk, showing concentric layers; a', its semi-fluid center;…

Cross-section of an egg, showing the different parts.

Cross-section of an egg

Cross-section of an egg, showing the different parts.

A scene from the Middle Ages, in which two people are plowing their land. One drives the oxen with a whip, while the other guides the plow. The original image is from a 14th century manuscript.

Plowing in the Middle Ages

A scene from the Middle Ages, in which two people are plowing their land. One drives the oxen with a…

Diagram showing the three successive stages of development. Transverse vertical sections. The yolk sac, ys, is seen progressively diminishing in size. In the embryo itself the medullary canal and notochord are seen in section. a', in middle figure, the alimentary canal, becoming pinched off, as it were, from the yolk sac; a', in right hand figure, alimentary canal completely closed; a, in last two figures amnion; ac', cavity of amnion filled with amniotic fluid; p, space between amnion and chorion continuous with the pleuroperitoneal cavity inside the body; vt, vitelline membrane; ys, yolk sac, or umbilical vesicle.

Development of the Yolk Sac

Diagram showing the three successive stages of development. Transverse vertical sections. The yolk sac,…

Diagram of the various stages of cleavage of the yolk.

Stages of Cleavage of the Yolk

Diagram of the various stages of cleavage of the yolk.