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FIG. 6. SURFACE VIEWS OF THE EARLY STAGES OF THE SEGMENTATION IN A FOWL'S EGG. (A and G after Coste.)

A represents the earliest stage. The first furrow (b) has begun to make its appearance in the centre of the germinal disc, whose periphery is marked by the line a. In J3 } the first furrow is completed nearly across the disc, and a second similar furrow at right angles to the first has appeared. The disc thus becomes divided somewhat irregularly into quadrants by four (half) furrows. In a later stage ((7) the meridian furrows b have increased in number, from four, as in B, to nine, and cross furrows have also made their appearance. The disc is thus cut up into small central (c) and larger peripheral (d) segments. Several new cross furrows are seen just beginning, as ex. gr. close to the end of the line of reference d.

Viewed from above, a furrow is seen to make its appearance, running across the germinal disc, though not for the whole breadth, and dividing it into two halves (Fig. 6, A). This primary furrow is succeeded by a second at right angles to itself. The surface thus becomes divided into four segments or quadrants (Fig. 6, B).

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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