1 70 THE OLFACTORY ORGANS AND THE OLFACTORY LOBES. 



macht es wahrscheinlich dass wir es hier mit einem neuen Organ zu thun haben" 

 applies equally well to the lateral olfactory nerve of Limulus. 



The nervus terminahs in elasmobranchs may have either a neural or a haemal 

 origin, but it is generally closely connected with the lamina terminalis (Locy); 

 or according to Pinkus in Protopterus, it "Am vorderende des recessus praeopticus 

 das Zweischenhirn verlast," thus indicating its probable origin near the root of 

 the lateral eye ganglion, as opposed to the origin of the main olfactory from the 

 dorsal anterior surface of the hemispheres. 



4. Origin of Olfactory Ganglia. — Arthropods. The lateral placode is 

 a primitive visual organ which becomes bodily converted into the giant 

 ganglion cells of the lateral nerves. The median placode is retained to form 

 the epithelial area in or near which all the nerves terminate. Its ganghon 

 cells are very minute and arise as outgrowths of the hemispheres and of the 

 olfactory lobes. 



Vertebrates. The difference between the development of the median and 

 general placodes is unknown. 



5. Position of Placode Cells. — Arthropods. The olfactory placodes 

 arise from the anterior lateral margin of the open medullary plate, but unlike the 

 adjacent visual placodes they are not swept into the neurocoele by the overgrowth 

 of the paUal fold; consequently the sensory epithehum is upright, and does not 

 form the wall of a closed sac. 



Vertebrates. The same. 



6. Serial Location of the Placodes and their Migration.— In arthropods 

 (Limulus), the lateral olfactory placodes are originally located on the margins of 

 the medullary plate (procephalic lobes), between the median ocelli and the lateral 

 eyes; they therefore appear to form the second set of cranial sense organs and 

 nerves; the median ocelli forming the first set, and the lateral eyes, the third 

 (Fig. 142). The lateral olfactory placodes first move toward the anterior 

 median margin of the palial fold (edge of the neuropore) and then forward, 

 taking up a position in the adult either on the neural surface (Limulus) , the apex 

 (Branchipus), or the haemal surface of the head (many phyllopods), its position in 

 each case being determined by local variations in the growth of the forebrain and 

 the external surface of the forehead. The arrangement of ocelli, olfactory organs, 

 and lateral eyes in the fully formed head, may, or may not, agree with their primi- 

 tive serial arrangement on the margins of the cephalic lobes. The olfactory 

 organs may stand alone in the adult (Limulus) or they may unite with the ocelli 

 and lateral eyes to form a compact median group (Apus, Limnadia etc.). (Figs. 

 8 and in.) 



In vertebrates the same conditions are indicated, but the serial order of the 

 ocellar, olfactory, and lateral eye placodes cannot be certainly determined in 

 vertebrates without locating their positions on the margins of the open neural 

 plate more accurately than has yet been done. Their serial order on the surface 

 of the head in the later stages is not decisive. 



