just mentioned, a couple of irregular openings represent the anterior end of the bulbus arteriosus, posterior and ventral to which is the heart (_ht_), a large, irregular cavity. The dorsal aorta (_ao_) may be seen as an elongated opening in the mesoblast, extending in this section from the middle region of the pharynx to the posterior end of the figure where it is somewhat torn. Two of the eighteen or twenty pairs of mesoblastic somites possessed by this embryo are shown at the posterior end of the figure (_s_), where the plane of the section was far enough from the median line to cut them. STAGE XI FIGURE 14 (PLATE XXI.) Only the anterior region of this embryo is shown in the figure, which is a ventro-lateral view. While there is some change in the general shape and in parts of the head, the reason for figuring this stage is to show the first gill cleft (_g′_), which lies at an acute angle to the long axis of the neck behind the eye (_e_). The cleft is narrow but sharp and distinct in outline; it shows neither in this nor in the following stages the branched, Y-shaped outline mentioned by Clarke. STAGE XII FIGURES 15-15_f_ (PLATES XXI., XXII.) In this stage, also, only the anterior region of the embryo is figured in surface view. The shape of the head is about the same as in the preceding stage, but it is drawn in exact profile. Three gill clefts (_g¹⁻³_) are now present, and are wide and distinct. The first cleft, as in the preceding stage, lies at an acute angle to the long axis of the pharynx and nearly at right angles to the second cleft. The third cleft sends a wide branch (_g⁴_) toward the posterior, as has been described by Clarke, from which, or in connection with which according to Clarke, the fourth cleft will develop. All three clefts may be distinctly seen to open entirely through the pharyngeal wall. The outlines of the visceral folds, especially of the mandibular, begin to be apparent. The nasal pit (_n_) now shows as a round depression in front of the more definitely outlined eye (_e_). The auditory vesicle (_o_) is so deep beneath the surface that it may be seen only by transmitted light. Figures 15_a_-_e_ represent transverse sections of an embryo of about this general state of development, except that the gill clefts are not so definitely open as in the surface view.