STRUCTURE OF THE VERTEBRATES 303 



The olfactory epithelium arises on the dorsal side of the 

 embryonic head, and the region soon pits in to form olfactory 

 sacs. The deeper layer of cells develops short dendrites which 

 push between the supporting cells toward the lining of the sac; 

 and axones which grow backward toward the olfactory lobe, 

 thus forming the olfactory nerve. 



The cyclostomes develop a single naso-pituitary pouch. The 

 paired olfactory sacs and nerves lie near the dorsal region of 

 the tube. The pituitary portion of the tube extends posteriorly 

 to the level of the hypophysis of the brain, ending blindly in 

 the lampreys, but opening into the mouth in the myxinoid 

 cyclostomes. 



The olfactory pits are paired in the fish and all higher verte- 

 brates, with the pituitary pouch arising separately from the 

 stomodeal region. As the chondrocranium develops the sacs are 

 pushed anteriorly, and ventrally in fish with a rostrum. The 

 sacs end blindly in the fish, and serve only an olfactory function. 



In the amphibia the sacs have an origin similar to that of the 

 fish, but the invaginations push ventrally toward the mouth 

 cavity, and break through to form the internal nares, or choanae. 

 The olfactory epithelium lies near the outer end of the tubes, 

 the external nares, although accessory patches of sensory epi- 

 thelium are near the internal opening. (This internal organ 

 degenerates in higher vertebrates, and is degenerate and non- 

 functional in the human). 



The conditions in the early reptiles are hardly changed, with 

 the exception of the development of lateral projections into the 

 nasal passage. These conchae are homologous with the more 

 highly developed turbinals of the mammals. The Crocodilia 

 have more highly developed conchae, and in this order of rep- 

 tiles the maxillary and palatine bones grow medially to form a 

 hard palate, thus elongating the nasal passage so that the in- 

 ternal nares are at the back of the mouth near the pharynx. 



There is a great increase in the sensory area in the mammals, 

 although the olfactory lobes are relatively much smaller than 

 in more primitive vertebrates, particularly the fish. The sensory 

 epithelium is accommodated by the growth of turbinals, bony 

 scrolls projecting into the nasal passage. The more anterior 

 naso- and maxillo-turhinals, arising from the nasal and maxillary 



