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Fig. 3. Four-weeks (28 days) horse embryo and its foetal appendages

A semi-diagrammatic representation of a four-weeks' (28 days) horse embryo and its foetal appendages. The embryo, which is curved so that the tail lies under the head, measures nearly three-eighths of an inch in length. The limbs are represented by lobes entirely composed of cells, i.e., the rudiments of the limb skeleton have not yet appeared. Behind the head are three arches and three clefts, but the clefts do not appear to open, as in fishes, into the pharynx. The amnion (am.) surrounds the embryo, and two stalks proceed from the under surface. The stalk proceeding to the left side connects the embryo with the yolk sac (y.s.), the stalk passing to the right contains the vessels of the allantois.

The allantois {all.) is already in contact with the embryonic sac (d), and with the amnion, and it has many vessels (v l ) in its wall. The yolk sac is vascular (v.), as far as the circular blood-vessel (s.t.), and crowded with granules which have entered by the absorbing area (a, b, c). The cells of the outer tunic at t.g. (on a level with the growing point of the allantois) have undergone considerable elongation, while the cells in the area {a, b, r) have given rise to a number of irregular ridges and processes.

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Reference

Ewart, J.C. A Critical Period in the Development of the Horse. London: Adam and Charles Black (1897).