The reader will ask: "Where are the mouth and the anus?" These are not at first present in the embryo. The whole of the primitive gut-cavity is completely closed, and is merely connected in the middle by the vitelline duct with the equally closed cavity of the embryonic vesicle (Figure 1.140). The two later apertures of the alimentary canal--the anus and the mouth--are secondary constructions, formed from the outer skin. In the horn-plate, at the spot where the mouth is found subsequently, a pit-like depression is formed, and this grows deeper and deeper, pushing towards the blind fore-end of the capital cavity; this is the mouth-pit. In the same way, at the spot in the outer skin where the anus is afterwards situated a pit-shaped depression appears, grows deeper and deeper, and approaches the blind hind-end of the pelvic cavity; this is the anus-pit. In the end these pits touch with their deepest and innermost points the two blind ends of the primitive alimentary canal, so that they are now only separated from them by thin membranous partitions. This membrane finally disappears, and henceforth the alimentary canal opens in front at the mouth and in the rear by the anus (Figures 1.141 and 1.147). Hence at first, if we penetrate into these pits from without, we find a partition cutting them off from the cavity of the alimentary canal, which gradually disappears. The formation of mouth and anus is secondary in all the vertebrates. (FIGURE 1.148. Longitudinal section of the fore half of a chick-embryo at the end of the first day of incubation (seen from the left side). k head-plates, ch chorda. Above it is the blind fore-end of the ventral tube (m); below it the capital cavity of the gut. d gut-gland layer, df gut-fibre layer, h horn plate, hh cavity of the heart, hk heart-capsule, ks head-sheath, kk head-capsule. (From Remak.))