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 (Fig. 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 cœlom-folds. A narrow cleft in the latter indicates the body-cavity (_uwh_); this separates the two plates of the cœlom-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. Fig.133. Human embryo at the sandal-stage. Fig. 134. Sandal-shaped embryonic shield of a rabbit of nine days. Fig. 133—Human embryo at the sandal-stage, one-twelfth of an inch long, from the end of the second week, magnified. (From _Count Spee._) Fig. 134—Sandal-shaped embryonic shield of a rabbit of nine days. (From _Kölliker._) (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 vertebræ).