FRONTAL SECTIONS OF HEAD, EMBRYO OF 20 MM. 327 



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. 220). 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- 

 fibers arise from nerve-cells either of the central nervous system or of the gan- 

 glia. Morphologically, therefore, the olfactory nerve takes a unique place, and 

 is not directly comparable with any other nerve of the brain. The cells which 

 accumulate in the course of the olfactory nerve do not, so far as known, have any 

 direct share in the production of the nerve-fibers; nor do they result in the 

 formation of the medullary sheaths, as they do in other nerves, the olfactory 

 nerve-fibers remaining naked, as it is termed, throughout life.. 



Section through the. Fore-brain and Eyes (Fig. 221). The section passes behind 

 the nasal cavities, no part of which is shown. The maxillary and mandibular 

 processes are united and the pharynx, Ph, appears as a closed cavity. On the 

 dorsal side of the section the fore-brain stands out conspicuously, both from its 

 dark staining and from being surrounded by the lightly stained broad zone of the 

 arachnoid, arach. The cavity of the fore-brain has two lateral expansions, L.V, 

 the lateral ventricles, which extend outward and upward. The walls, H, of the 

 lateral ventricles are much thinner than the walls of the lower part of the fore- 

 brain and are the anlages of the cerebral hemispheres. In the median plane the 

 hemispheres include between themselves a partition, Fx, of mesodermic tissue 

 which may be designated as the embryonic falx, since within it, though considerably 

 later, the adult falx will be differentiat d. In the adult, the falx appears as a 

 prolongation of the dura mater. From the bottom of the falx there extends on 



