forms a very striking feature of the section. Dorsal to the posterior end of the Wolffian body is a small, oval mass of very fine tubules (_k_), which do not stain so darkly as do the Wolffian tubules; this mass is apparently the beginning of the permanent kidney, the metanephros. Its tubules, though their origin has not been determined, seem to be entirely distinct from the tubules of the Wolffian body. A single vertical section through the anterior part of the head of an embryo of this age has been represented in Figure 23_b_. On the right side the plane of the section cut through the lens of the eye (_ln_); on the left side the section was anterior to the lens. The upper (_ul_) and lower (_ll_) eyelids are more evident here than in the surface view. Owing to the hardness of the lens, its supporting structures were torn away in sectioning. The vitreous humor is not represented in the figure. The superior (_ur_) and inferior (_lr_) recti muscles are well shown on the right side; they are attached to the median part of a Y-shaped mass of cartilage (_se_), which may be termed the sphenethmoidal cartilage. Between the branches of this Y-shaped cartilage the anterior ends of the cerebral hemispheres (_ch_)--better called, perhaps, the olfactory lobes--are seen. Between the lower end of the sphenethmoidal cartilage and a dorsally evaginated part of the pharynx are two small openings (_pn_); when traced forward these tubes are found to open into the convoluted nasal chamber, while a short distance posterior to the plane of this figure they unite with each other and open almost immediately into the pharynx. The rather complicated structures of the nasal passages of the alligator have been described by the writer in another paper (57). In the lower jaw the cartilage (_mk_) is seen on either side and several bands of muscle are developing in the mesoblast. Two deep grooves give form to what may be called the rudimentary tongue (_tn_). In both jaws one or two tooth rudiments (_to_) may be distinguished as small invaginations of ectoderm. STAGE XXI FIGURE 24 (PLATE XXVII.)