256 



STUDY OF PIG EMBRYOS. 



fibers arise from nerve-cells either of the ventral 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, as it is termed, naked throughout life. 



arach. Sk. 



Fig. 147. — Pig, 20.0 mm. Frontal Section of Head. Series 40, Section 84. 

 arach, Arachnoid membrane. H, Cerebral hemispheres. Max.tb, Maxilloturbinal fold. Mdb, Mandible. Mx. 

 sup, Superior maxillary nerve. Nas.tb, Naso-turbinal fold. N.olf, Olfactory nerve. Sept, Cartilaginous 

 septum of the nose. Sk, Mesenchymal anlage of the dura mater and skull. Ton, Tongue. X ^^ diams. 



Section through the Fore-brain and Eyes (Fig. 148). — The section passes 

 behind the nasal cavities, no part of which is shown. The maxillary and mandi- 

 bular 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, 



