anterior cupola 



external nasal 

 opening 



nasal tectum 

 nasal septum 



paranasal cartilage cerebrol lobes 



B 



nasal passage 



Jacobson's organ 



H 



Figure 13-19. Two stages in the development of the nasal passage and Jacobson's organ in the 

 lizard. A to E, serial sections through embryo of 4-mm heod length; F to H, serial sections of em- 

 bryo of 4.5-mm head length. (After Bellairs, 1950) 



lack turbinals. In the fishes including the dipnoans and 

 Lalimeria, there is a highly developed olfactory organ — usu- 

 ally a rosette of folds extending into the nasal chamber. 



The development of the olfactory sense varies; some ani- 

 mals have it highly developed, while others do not. 



Jacobson's organ 



Tetrapods Jacobson's organ is found in all tetrapods and 

 is associated with a distinct vomeronasal nerve, a division 

 of the olfactory nerve, and an accessory olfactory bulb 

 within the olfactory bulb. 



In the amphibians a groove or pocket, the lower blind 

 sac, in the lateral or anterolateral wall of the nasal cham- 

 ber has an area of sensory epithelium which is identified as 

 the Jacobson's organ. Typically Jacobson's organ, and its 

 pouch, lies medially (Figure 13-19); the position in the am- 

 phibian appears to be the results of rotation of the nasal 

 chamber during development. The olfactory sensory epithe- 

 lium lines much of the dorsal, medial, and ventral walls of 

 the nasal passage. Throughout this sensory area there are 

 many flask-shaped Bowman's glands, or glandulae olfac- 

 toriae. The sensory epithelium of the Jacobson's organ lacks 

 these glands. 



An accessory olfactory bulb is present in amphibians, but 



this lies dorsolaterally (Hypogeophis) or ventrolaterally (Rana) 

 to the posterior end of the bulb, whereas in amniotes it is 

 usually located posteromedialiy — dorsally in turtles and 

 most mammals, dorsolaterally in two shrews (Blarina and 

 Scalopus). Among the Amphibia, a distinct vomeronasal 

 nerve has been reported only in Rana. 



In the turtle the ventral or anteroventral part of the nasal 

 passage, ventral to the olfactory chamber, has areas of sensory 

 epithelium served by a vomeronasal (medial) division of the 

 olfactory nerve. In other reptiles a pocket forms in the ven- 

 tromedial wall of the nasal pit well before the nasal proc- 

 esses separate the external naris from the internal choana. 



The anlage of this organ disappears in the crocodilian, 

 and in birds, but in Sphenodon and the squamates (lizards 

 and snakes) it enlarges to form a tubular or spherical pocket 

 in the medial nasal wall. In squamates the duct of this 

 pocket moves down and eventually comes to open separately 

 into the mouth; it retains its embryonic relationship in 

 Sphenodon. 



In mammals this organ is always present in the embryo 

 but may be lacking in the adult (man, bats, and whales); 

 when present it is associated with the nasopalatine duct 

 (incisive or Steno's duct). In rodents and rabbits this organ 

 opens into the nasal passage, not into the mouth. Bowman's 

 glands are present in the olfactory epithelium but absent 

 in Jacobson's organ — this is always the case in amniotes. 



SENSORY ORGANS 



403 



