A horizontal section through the anterior end of an embryo of this age is shown in Figure 15_f_. While enclosed of course in the same membranous amnion (_a_), the pharyngeal region of the section is separated by a considerable space from the more anterior region where the section passes through the forebrain (_fb_) and eyes. The spinal cord (_sc_), notochord (_nt_), muscle plates (_mp_), aortæ (_ao_), and anterior cardinal veins (_ac_) need no special description. The appearance of the pharynx (_ph_), with its gill clefts and folds, is quite similar to that of the corresponding structures in the chick. None of the four clefts (_g¹⁻⁴_) show, in the plane at which the section was cut, any connection with the exterior; in fact the fourth cleft (_g⁴_) would scarcely be recognized as a cleft if seen in this section alone. One or two of the more anterior clefts are open to the exterior. Three pairs of aortic arches are seen, and each visceral fold has a central condensation of mesoblast. STAGE XIII FIGURES 16-16_g_ (PLATES XXII., XXIII.) The embryo (Fig. 16) now lies on one side, body torsion being complete. The curvature of the body is so marked that the exact length is difficult to determine. The eye (_e_) and ear (_o_) have about the same superficial appearance as in the preceding stage. The nose is not shown in this figure. About thirty somites are present; the exact number cannot be determined in surface view. The amnion is complete, though not shown in the figure, and the tail (_t_) is well formed. The umbilical stalk was torn in the removal of the embryo, so that it is not shown in the figure. The dim outline of the now convoluted heart may be seen if the “cleared” embryo be viewed by transmitted light; it is not shown in the figure. The allantois (_al_) is a rounded sac of considerable size just anterior to the tail. Four gill clefts (_g¹⁻⁴_) are now present; the most posterior one is more faint than is represented in the figure, and it could not be definitely determined from a surface view whether or not it opened to the exterior. The mandibular fold (_md_) is now fairly well outlined, but there is as yet no sign of the maxillary process. Figure 16_a_ is the most anterior of a series of transverse sections made of an embryo of the approximate age of the surface view just described; it passes through the tip of the forebrain (_fb_) and shows the nasal pit (_n_) of the right side. The great thickening of ectoderm in the region of the nasal invagination is represented by a solid line. Owing to the obliquity of the section, the left nasal pit was not cut. The mesoblast is quite dense and contains two or three small blood-vessels near the roof of the brain. The plane of this section, owing to the cranial and body flexure, cut the embryo also in the region of the pharynx; this part of the section was, as a matter of convenience, omitted from the drawing.