A sagittal section of the entire embryo (except the tail) of this age is shown in Figure 23_a_. In the head region the section is nearly median, while the posterior part of the body is cut slightly to one side of the middle line. At the tip of the now well-developed snout is seen one of the nostrils (_an_), cut through the edge; its connection with the complicated nasal chamber (_n_) is not here seen, nor is the connection of the nasal chamber with the posterior nares (_pn_). The pharynx (_ph_), is anteriorly connected with the exterior through the mouth (_m_) and the nares, while posteriorly it opens into the œsophagus (_oe_); the trachea (_ta_), though distinct from the œsophagus, does not yet open into the pharynx. In the lower jaw two masses of cartilage are seen, one near the symphysis (_mk_) and one near the wall of the trachea, doubtless the rudiment of the hyoid. The deep groove back of the Meckel’s cartilage (_mk_) marks the tip of the developing tongue, which here forms the thick mass on the floor of the mouth cavity. Dorsal to the pharynx a mass of cartilage (_se_) is developing in the sphenethmoid region. This being a median section, the ventricles of the fore- (_fb_), mid- (_mb_), and hindbrain (_hb_) are seen as large cavities, while the cerebral hemispheres (_ch_) appear nearly solid, only a small portion of a lateral ventricle showing. The paraphysis (_epi_) is cut a little to one side of the middle and so does not show its connection with the brain. At the base of the brain the infundibulum (_in_) is seen as an elongated cavity whose ventral wall is in close contact with a group of small, darkly staining alveoli (_p_), the pituitary body. Extending posteriorly from the pituitary body is a gradually thickening mass of cartilage (_bp_), which surrounds the anterior end of the notochord (_nt_) and may be called the basilar plate. In its anterior region, where the section is nearly median, the spinal column shows its canal, with the enclosed spinal cord, while toward the posterior end of the figure the vertebræ are cut to one side of the middle line, and hence show the neural arches (_na_) with the alternating spinal ganglia (_sg_). Near the posterior end of the figure the pelvic girdle (_pl_) is seen. The largest organ of the embryo, as seen in this section, is the heart, of which the ventricle (_vn_) seems to be closely surrounded, both in front and behind, by the auricles (_au_). The liver (_li_) is the large, reticular mass back of the heart. Dorsal and anterior to the liver is the lung (_lu_), now of considerable size and development. The enteron is cut in several places (_oe_, _i_) and its walls are beginning to show some differentiation, though this cannot be seen under the magnification here used. One of the Wolffian bodies is seen as a huge mass of tubules (_wt_) extending from the pelvic region, where the mass is greatest, to the region of the lungs. The Wolffian tubules stain darkly and the whole structure