OLFACTORY ORGAN 



261 



becomes grooved : its unpaired character is probably secondary. 

 In Petromyzon it arises on the ventral side of the head in front of 

 the oral involution (stomodaeum), and between it and the mouth is 

 another ectodermal invagination, the pituitary sac (Fig. 190). In 

 the course of development the olfactory and pituitary imaginations 

 become sunk in a common pit, which, owing to the growth of the 

 large oral funnel, gradually becomes shifted to the dorsal side. On 

 the further elongation of the naso-pituitary sac to form the above- 

 mentioned tube, the olfactory sac opens into it posteriorly, and is 

 incompletely divided into right and left halves by a septum which 



Hyp HO 



-.-Chios 



VOD 



Fi<;. 190, C. 



grows down from the dorsal side. The pituitary body arises by 

 the formation of follicles from the pituitary sac where it passes 

 below the infundibulum. 



Fishes. The position of the olfactory organ in Elasmobranchs 

 (Fig. 191, A) differs from that seen in Cyclostomes in being on the 

 under instead of the upper surface of the snout, and thus retains 

 a more primitive position. In many forms each nostril is con- 

 nected with the mouth by a naso-oral groove (cf. Note on p. 259). 

 From these Fishes onward the organ is always paired, each sac 

 being more or less completely enclosed within a cartilaginous or 

 bony investment forming an outwork of the skull, and being situated 

 between the eye and the end of the snout, either laterally or more 

 or less dorsally. 



In the course of development each external nostril of Teleo- 

 >r i mes becomes completely divided into two portions, an anterior 

 and a posterior, by a fold of skin. The anterior aperture is often, 

 and the posterior sometimes, situated at the summit of a longer or 

 shorter tube, lined by ciliated cells, and the distance between the 



