THE ECTODERM AND ITS DERIVATIVES 



179 



Olfactory 

 tube 



Olfactory 

 epithelium 



Internal nores (choona) 



Lateral appendix Olfactory cartilage 



External and internal nares of the 11 mm. frog tadpole. (Left) External nares. 

 (Right) Internal nares (choana) opening into the pharynx. 



Pharynx 



the 2.5 mm. body length stage, long before the time of hatching. The 

 overlying superficial epithelial ectoderm disappears so that the nerv- 

 ous layer of the placode becomes* the exposed lining of the olfactory 

 pit. 



After hatching (6 mm. stage) a solid rod of ectodermal cells grows 

 ventro-laterally from the olfactory pit to become attached to the 

 pharynx just dorsal to the oral plate (i.e., stomodeum). By the 1 1 mm. 

 stage this core of cells acquires a lumen which is continuous from the 

 external nares (olfactory pits) to the internal nares (internal choanae) 

 which open into the pharynx. The major part of this olfactory tube is 

 lined with non-sensory epithelium. It develops a dorsal and a ventral 

 chamber (sacs), each of which acquires glandular masses which are 

 known as the organs of Jacobson. 



The neuroblasts of the olfactory placodes send extensions pos- 

 teriorly and give rise to the fibers which form the olfactory or first 

 cranial nerve. These grow toward the brain and are guided in their 

 directional development by the outgrowth of the telencephalon known 

 as the olfactory lobes. 



The Lateral Line Organs. 



The most posterior or fourth cranial placode (cranial nerve X) 

 sends a growth posteriorly beneath the lateral body epidermis, on either 

 side, beginning at about the 4 mm. stage. It grows posteriorly to the 



