208 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



This closes in, the edges of each groove uniting, so that a tube or duct 

 is formed, leading from the naris into the oral cavity, where an inter- 

 nal nans or choana occurs (fig. 218, ch). Later maxillary and pre- 

 maxillary bones arise ventral to the narial passage, so that the ducts 

 appear to run through the skull. The position of the choanae varies 

 considerably, being just inside the jaws in the amphibia and lower 

 reptiles, farther back in the higher reptiles and the birds and mam- 

 mals, the nasal passages being cut off from the roof of the primitive 

 mouth by the ingrowth of the palatal processes of the maxillary 

 bones and higher, by similar extensions of the palatines, and in some 

 cases, of the pterygoids (fig. 218). 



Incomplete closure of the oronasal groove results in the deformity known as 

 'hare-lip' externally, while 'cleft-palate' is the result of failure of palatines 

 and sometimes of maxillaries to meet below the nasal passages. 



Fig. 219. Fig. 220. 



Fig. 219. — Section through the nasal region of 5i>en, after Seydel. en, nasal cavity, 

 »g; Jacobson's gland; jo, organ of Jacobson; v, vomer. 



Fig. 220. — Section of nose of Chelonia cauana, after Gegenbaur. c, concha; ch, 

 choana; i, inner olfactory groove; n, projection of naris between dotted Unes. 



In the dipnoi the olfactory membrane forms a few large folds on 

 the dorsal side of the respiratory duct formed from the oronasal tube. 

 In the amphibia the sensory surface has a similar position on the 

 upper medial surface (fig. 219), with frequently a lateral pocket 

 lined with sensory epithelium, the beginnings of an organ of Jacobson. 

 In the same group glands (inner and outer Jacobson's glands) 

 occur for moistening the olfactory epithelium. Usually there is 

 little complication of the olfactory surface, but in a few urodeles 

 (Plethodon) there is a projection from the lateral wall, the first indi- 

 cation of the conchae which acquire such development in the higher 

 groups. There is frequently a differentiation of the nasal passage in- 

 to a ventral respiratory duct lined with ordinary and a more dorsal 

 olfactory duct lined with sensory epithelium. In the lower urodeles 

 the diverticulum representing the organ of Jacobson is on the medial 



