582 THE EMBRYOLOGY OF THE FROG , 



ganglion. It is from these cells that the sheath cells of the true olfactory 

 nerves (I cranial nerves) seem to be formed, the olfactory nerves them- 

 selves forming from the sensory cells of the olfactory placode. 



The olfactory pits are often said to form the anterior nares, while 

 the openings from the anterior nares into the mouth cavity are called 

 the posterior nares, internal nares, or choanae, all three terms meaning 

 approximately the same thing. 



The olfactory development is quite complicated and cannot be 

 worked out in all details by the student in an elementary course such as 

 this, but it is important that the main features be understood, so that 

 light may be thrown upon later studies. 



The epithelial lining of the olfactory pits forms a cavity on the dor- 

 sal side soon after hatching. This cavity then closes to form a separate 

 dorso-lateral lobe, which disappears entirely as metamorphosis con- 

 tinues. 



During metamorphosis, various thickenings and outpocketings ap- 

 pear in the olfactory organ, and there is a sharp bend in the main axis. 

 The most important of the outgrowths is an extension from the ventral 

 side of the olfactory chamber, where a solid mass of cells proliferates. 

 This extended portion then acquires a cavity, grows rapidly, and turns 

 transversely toward the medial side. This structure is to become Jacob- 

 son's organ (Fig. 339). A large glandular mass develops upon the medial 

 end of this organ. 



Opposite Jacobson's organ another growth appears which is non- 

 nervous, that is, is not lined with nerve cells. This becomes a large sac, 

 and the cavity of the sac is then added to the olfactory chamber. Still 

 another growth appears anteriorly, close to the base of the olfactory 

 duct. It is into this latter structure that the duct from the lachrymal 

 glands enters. Still later (about the time of metamorphosis) a dorsal sac 

 grows out from the medial and posterior walls of the tube. 



During the late metamorphosis, the axis of the olfactory organ is 

 sharply bent by the shifting of the internal nares, and other glands ap- 

 pear as outgrowths, both in the olfactory chamber and in the posterior 

 walls of the internal nares. 



THE SENSE ORGANS OF THE LATERAL LINE (Fig. 340) 



In all gill-bearing animals, and in all animals that have gills at any 

 time during their development, such as the frog, a series of sensory 

 drgans develop which are known as lateral line organs. These organs 

 vary to a very considerable extent, but three or four of them are rather 

 constant. These are: (1) the supraorbital line, which runs forward from 

 the ear region over the eye to the tip of the snout. Twigs from the 

 ophthalmic branch of the VII cranial nerve innervate it, 



(2) The infraorbital line, which also runs from the ear region, but 



