In order to get a clear idea of this, we must understand carefully the relation of the embryonic shield to the germinative area and the embryonic vesicle. This is done best by a comparison of the five stages which are shown in longitudinal section in Figs. 138–142. The embryonic shield (_c_), which at first projects very slightly over the surface of the germinative area, soon begins to rise higher above it, and to separate from the embryonic vesicle. At this point the embryonic shield, looked at from the dorsal surface, shows still the original simple sandal-shape (Figs. 133–135). We do not yet see any trace of articulation into head, neck, trunk, etc., or limbs. But the embryonic shield has increased greatly in thickness, especially in the anterior part. It now has the appearance of a thick, oval swelling, strongly curved over the surface of the germinative area. It begins to sever completely from the embryonic vesicle, with which it is connected at the ventral surface. As this severance proceeds, the back bends more and more; in proportion as the embryo grows the embryonic vesicle decreases, and at last it merely hangs as a small vesicle from the belly of the embryo (Fig. 142 _ds_). In consequence of the growth-movements which cause this severance, a groove-shaped depression is formed at the surface of the vesicle, the _limiting furrow,_ which surrounds the vesicle in the shape of a pit, and a circular mound or dam (Fig. 139 ks) is formed at the outside of this pit by the elevation of the contiguous parts of the germinal vesicle. Fig.136. Transverse section of the embryonic disk of a chick at the end of the first day of incubation. Fig. 136—Transverse section of the embryonic disk of a chick at the end of the first day of incubation, magnified. The edges of the medullary plate (_m_), the medullary swellings (_w_), which separate the medullary from the horn-plate (_h_), are bending towards each other. At each side of the chorda (_ch_) the primitive segment plates (_u_) have separated from the lateral plates (_sp_). A gut-gland layer. (From _Remak._)