FRONTAL SECTIONS OF HEAD, EMBRYO OF 20 MM. 255 



extending toward the median line. From the oral fissure there extend upward 

 two irregular cavities, the nasal chambers. The two cavities are separated from 

 one another by a broad mass of tissue, the nasal septum, the ventral edge of which 

 at this stage forms a portion of the roof of the mouth-cavity. In the center of the 

 nasal septum is a broad band, Sept, of denser mesenchymal tissue, the anlage of 

 the cartilaginous septum of the nose. On either side of the nasal septum are the 

 two irregularly shaped nasal cavities, which open into the mouth between the ven- 

 tral edge of the nasal septum and the inner edge of the maxillary process. The 

 medial side of each nasal cavity is comparatively regular, but the external side 

 shows two prominences, each of which is formed by a mass of mesenchymal tissue 

 covered by epithelium. The upper of these projections, nas. tb, is the anlage of 

 the naso-turbinal fold, and the lower, max. tb, the anlage of the maxillo-turbinal 

 fold. In the nasal septum itself are two oval rings of epithelium, sections of 

 Jakobson's organs. This organ is an evagination of the epithelial lining of the 

 nasal cavity which opens anteriorly and extends backward some distance in the 

 nasal septum. In the maxillary process may be observed the maxillary nerve. 

 The number of cells in the nerve has increased, and consequently the division of 

 the nerve-fibers into distinct bundles has become more marked as compared with 

 the pig embryo of 12 mm. 



Section through the Middle of the Snout (Fig. 147). The relations are very 

 similar to those described in the previous section, so that it will suffice to note the 

 three most important differences : First, the absence of Jakobson's organ ; second, 

 the appearance of the tongue, Ton, and third, of the olfactory nerve, N. olf. The 

 tongue is a protuberance attached to the lower jaw, Mdb. Its connection with 

 the jaw is rather narrow and corresponds to the frenum. The tongue extends 

 upward between the maxillary processes until it is almost or quite in contact with 

 the lower edge of the nasal septum. It is formed by a somewhat dense mass of 

 tissue in which there is no very evident histological differentiation, and is covered 

 by a layer of epithelium of moderate thickness and which is probably entirely 

 derived from the entoderm, for the tongue first appears as a small median pro- 

 tuberance on the ventral floor of the pharynx, between the first gill pouches. 

 The olfactory nerve, N. olf, can be seen joining the lower part of the inner side of 

 the brain-wall and extending down toward the nasal cavity and branching. 

 Under the part of the nerve near the brain- wall numerous cells are mingled with 

 the fibers, and by their crowding render the nerve conspicuous in stained sections. 

 The fibers of the olfactory nerve differ from all other nerve-fibers known in ver- 

 tebrates. They arise as prolongations of certain of the epithelial cells of the 

 olfactory region of the nose and grow from these cells into the brain, where they 

 have their termination in the glomeruli of the olfactory bulb. All other nerve- 



