The spinal cord (_sc_) and notochord (_nt_), with the faintly outlined condensations of mesoblast in their region, need no special description. The pharynx (_ph_) is here reduced to an irregular, transversely elongated cavity, the lateral angles of which are connected on each side with the exterior through a tortuous and almost closed gill cleft (_g_), which must be followed through many sections before its inner and outer openings may be determined. Dorsal to the pharynx numerous blood-vessels (_bv_), both large and small, may be seen, while ventral to it is noticed a faint condensation of mesoblast (_la_), in the form of an inverted T, the anlage of the laryngeal structures. The ventral portion of the figure is made up of a nearly circular thin-walled cavity, the pericardium (_pr_). Most of the pericardial cavity is occupied in this section by the thick-walled ventricle (_vn_), above which is the bulbus (_b_) and the tip of the auricle (_au_). The bulbus is nearly circular in outline, though its cavity is very irregular. A few sections anterior to this, the opening of the bulbus into the ventricle is seen. In Figure 20_g_ the section represented is only a short distance posterior to the one represented by Figure 20_f_. The mesoblastic structures in the neighborhood of the spinal cord (_sc_) and notochord (_nt_) will be described in connection with the next figure, where they are more clearly defined. The œsophagus (_oe_)--or posterior end of the pharynx, whichever it may be called--is here a crescentic slit, with its convex side upward; ventrally it opens by a narrow glottis into the trachea (_ta_). The trachea is surrounded by the same condensed area of mesoblast (_la_) that was mentioned in connection with the preceding figure, but the condensation is here more marked. From the bulbus (_b_) an aortic arch (_ar_) extends upward for a short distance on the right side, while to the left of the œsophagus an aortic arch (_ar_) is cut through the upper part of its course. Ventral to the bulbus the ventricle (_vn_) and two auricles (_au_) are seen surrounded by the pericardial wall.