Figure 20_h_ is in the region of the liver (_li_), which has about the same position in relation to the auricles (_au_) that was occupied by the ventricle in the last figure. The auricles are connected with each other by a wide passage. The trachea (_ta_) and the œsophagus (_oe_) are entirely distinct from each other; the former is a small, nearly circular hole, while the lumen of the latter is obliterated and its walls form a solid, bow-shaped mass of cells. Since there is a narrow space between this mass of cells and the surrounding mesoblast, it might be thought that the lumen of the œsophagus had been closed by the simple shrinkage of its walls; higher magnification, however, fails to show any sign of a collapsed lumen. It is doubtless the problematic and temporary closure of the œsophagus that is noticed in other forms. On each side of the œsophagus, in close relation with the anterior cardinal vein (_ac_), is noticed a nerve (_cn_) cut through a ganglionic enlargement. When traced forward these nerves are seen to arise from the region of the medulla, and when followed caudad they are found to be distributed chiefly to the tissues surrounding the newly formed bronchi; they are doubtless the tenth cranial nerves. On the right side of the figure the close connection of this nerve with the near-by gill cleft is seen. Above the paired aortæ (_ao_) the sympathetic nerves (_sy_) will be noticed. The mesoblast surrounding the spinal cord (_sc_) and notochord (_nt_) is distinctly condensed (more so than the figure shows) to form what may be called the centrum (_c_) and neural arch (_na_) of the vertebræ. The arch, owing to the slight obliquity of the section, shows here only on one side. The spinal cord is not yet completely enclosed by the neural arches. The muscle plates (_mp_) are in close connection with the rudiments of the vertebræ just mentioned. The spinal cord (_sc_) is here differentiated into three areas--a dense, deeply stained area immediately around the neurocœl; a less dense area of cells surrounding the inner area and extending ventralward as a rounded projection on each side; and an outer layer, with few or on nuclei, surrounding the inner two layers except on the dorsal side.