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Fig. 3. Section of a blastoderm of a fowl's egg at the commencement of incubation

The thin but complete upper layer ep composed of columnar cells rests on the incomplete lower layer I, composed of larger and more granular cells.

The lower layer is thicker in some places than in others, and is especially thick at the periphery.

The line below the under layer marks the upper surface of the white yolk. The larger so-called formative cells are seen at 6, lying on the white yolk. The figure does not take in quite the whole breadth of the blastoderm ; but the reader must understand that both to the right hand and the left ep is continued farther than I, so that at the extreme edge it rests directly on the white yolk.

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Reference

Foster, M., Balfour, F. M., Sedgwick, A., & Heape, W. (1883). The Elements of Embryology. (2nd ed.). London: Macmillan and Co.

The Elements of Embryology (1883)

Volume 1 The History of the Chick: Egg structure and incubation beginning | Summary whole incubation | First day | Second day - first half | Second day - second half | Third day | Fourth day | Fifth day | Sixth day to incubation end

Volume 2 The History of the Mammalian Embryo: General Development | Embryonic Membranes and Yolk-Sac | Organs from Epiblast | Organs from Mesoblast | Alimentary Canal | Appendix