n OLFACTOEY ORGAN 125 



pressions from without. Local concentrations of such sensory cells 

 and their further specialization for the better perception of some 

 particular type of stimulus has led to the evolution of the various 

 organs of special sense. 



The special sense organs of the vertebrate fall into two categories 

 — (I.) the organ of vision, perhaps the oldest organ of special sense, 

 which is developed within the limits of the central nervous system 

 and (II.) the other organs of special sense which have probably arisen 

 more recently from the sense cells of the skin outside the limits of 

 the central nervous system. 



As the organs belonging to the second category have evolved less 

 far from the primitive condition they will be considered first. They 

 appear to have become specialized functionally in two different 

 directions, those in the neighbourhood of the mouth for the apprecia- 

 tion of differences in chemical composition — the organs of taste and 

 smell — and those on other parts of the body surface for the apprecia- 

 tion of vibrations of the surrounding medium — the lateral line 

 organs and the organ of hearing. 



Olfactory Organ. — The olfactory organ arises in the form of a 

 localized thickening of the ectoderm on each side towards the anterior 

 end of the head. Later this thickened ectoderm becomes depressed 

 below the general surface so that it assumes a saucer- and later a 

 cup-shape; its external opening eventually becomes comparatively 

 narrow. 



In many of the Elasmobranch fishes the olfactory organ retains 

 throughout life the condition of a simple inpushing of the skin 

 opening to the exterior on the ventral side of the snout. In many 

 Vertebrates on the other hand characteristic changes come about in 

 the relations of the external opening. These will best be understood 

 by considering first what happens in the lung-fish Protopterus as 

 shown by Fig. 69. In A and B the olfactory organ is visible as a 

 little rounded dimple on each side. In C the dimple has become a 

 deep groove running obliquely from before backwards and outwards. 

 It is further seen that this groove is becoming involved in the 

 sinking in of the skin to form the buccal cavity. In D and E the 

 groove has become a deep slit narrow in its middle part and dilated 

 at its two ends. Finally, in the stage represented in F, the margins 

 of the narrow part of the slit ■ have undergone complete fusion so 

 that the continuous slit of the preceding stage is now represented 

 merely by its terminal portions which form two widely separated 

 rounded openings— the anterior and posterior nares (o// 1 and olf 2 ). 



Turning to the. other Vertebrates we find various divergences 

 from this simplest mode of origin of the external and internal nares 

 as seen in Protopterus and Geratodus. In the Actinopterygian fishes 

 the phenomena are quite similar to those described, only here 

 differential growth leads to the gradual shifting of the olfactory 

 organ and its openings from the ventral side of the snout up to its 

 dorsal side. The result is a topographical reversal of the positions of 



