It is all the more notable that the early separation of dorsal and ventral halves takes place in the same rigidly hereditary fashion in all the vertebrates. In both the acrania and the craniota the dorsal body is about this period separated from the ventral body. In the middle part of the body this division has already taken place by the construction of the chorda between the dorsal nerve-tube and the ventral canal. But in the outer or lateral part of the body it is only brought about by the division of the coelom-pouches into two sections--a dorsal episomite (dorsal segment or provertebra) and a ventral hyposomite (or ventral segment) by a frontal constriction. In the amphioxus each of the former makes a muscular pouch, and each of the latter a sex-pouch or gonad. These important processes of differentiation in the mesoderm, which we will consider more closely in the next chapter, proceed step by step with interesting changes in the ectoderm, while the entoderm changes little at first. We can study these processes best in transverse sections, made vertically to the surface through the sole-shaped embryonic shield. Such a transverse section of a chick embryo, at the end of the first day of incubation, shows the gut-gland layer as a very simple epithelium, which is spread like a leaf over the outer surface of the food-yelk (Figure 1.92). The chorda (ch) has separated from the dorsal middle line of the entoderm; to the right and left of it are the two halves of the mesoderm, or the two coelom-folds. A narrow cleft in the latter indicates the body-cavity (uwh); this separates the two plates of the coelom-pouches, the lower (visceral) and upper (parietal). The broad dorsal furrow (Rf) formed by the medullary plate (m) is still wide open, but is divided from the lateral horn-plate (h) by the parallel medullary swellings, which eventually close. (FIGURE 1.133. Human embryo at the sandal-stage, one-twelfth of an inch long, from the end of the second week, magnified twenty-five times. (From Count Spee.) FIGURE 1.134. Sandal-shaped embryonic shield of a rabbit of nine days. (From Kolliker.) (Back view from above.) stz stem-zone or dorsal shield (with eight pairs of primitive segments), pz parietal or ventral zone, ap pellucid area, af amnion-fold, h heart, ph pericardial cavity, vo omphalo-mesenteric vein, ab eye-vesicles, vh fore brain, mh middle brain, hh hind brain, uw primitive segments (or vertebrae).)