128 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



which a cavity makes its appearance secondarily, the actual involu- 

 tion of the surface being delayed or reduced or absent (Eig. 71). 



Sometimes, as is well shown in the case of Polypterus (Fig. 70, A), 

 the olfactory thickenings are at first continuous across the mesial 

 plane and this fact, taken in consideration with the fact that in the 

 Lampreys the olfactory organ of the adult is unpaired, obviously 

 suggests the possibility that the olfactory organ of Vertebrates in 

 general was originally unpaired (Kupffer). Though this must be 

 admitted as a possibility the evidence does not appear to be sufficient 

 to give the idea probability in view of the fundamentally paired 

 character of the portions of the brain associated with the olfactory 



organ, even in the case of the cyclo- 

 stomes where the organ as a whole has 

 an unpaired appearance. 



After the olfactory involution has 

 become definitely established it under- 

 goes various complications of form, 

 differing in detail in the various groups 

 but consisting for the most part of 

 bulgings outwards on the part of the 

 lining epithelium so as to bring about 

 an increase in its area. In the Elasmo- 

 branch these outgrowths take the form 

 of parallel grooves which gradually 

 become converted into deep slits 

 separated by thin partitions — the 

 Schneiderian folds. In Crossopterygians 

 instead of numerous folds with free 

 edges complete septa are formed which 

 radiate out from an axis formed by 

 the olfactory nerve and divide the 

 cavity as seen in transverse section 

 into distinct chambers, the lining of which in turn forms deep folds. 

 In the higher forms the outgrowths of the olfactory lining are fewer 

 in number and the projections left between them form the turbinate 

 which have characteristic arrangements in the different groups. 



Amongst the Reptiles a conspicuous development of the olfactory 

 apparatus is the Organ of Jacobson. This arises as a pocket-like 

 outgrowth of the lining epithelium, on its mesial side and near its 

 ventral edge, which becomes gradually constricted off from the 

 olfactory organ and opens into the buccal cavity in the region of the 

 posterior nares. In Chelonians, Crocodiles and Birds this organ has 

 disappeared except for a possible vestige in the form of a transient 

 bulging of the olfactory lining. 



A diverticulum which may correspond to Jacobson's organ makes 

 its appearance in Lung-fishes and Urodeles but in this case it becomes 

 gradually displaced outwards until it lies external to the olfactory 

 cavity. 



Fig. 71. — Longitudinal vertical sec- 

 tion through Polypterus (stages 

 28-29), showing the olfactory rudi- 

 ment as a thickening of tl)e deep 

 layer of the ectoderm in which a 

 cavity has developed secondarily. 



