OLFACTOEY OKGAN 



127 



nnMli}i.ii portion (median nasal process) and a lateral portion 

 (maxillary process). 



Tin- ridge hounding the olfactory involution on its outer side 

 iv 1 1 1, ( ins for a time separated by a distinct groove from the maxillary 

 process hut as the hittn- grows i or wan Is it obliterates this groove as 

 well~as the superficial portion of the cleft wliidi srp;irates it from 

 tin- iiii'di.in nasal process, the deep portion of the cleft remaining as 

 ,i definitive canal le.-iding from olfactory organ to huccal cavity. 



In Amphibians, as in Lepidosiren among the lung-fishes, the 

 posterior naris is frequently formed as a secondary perforation which 



Thai. 



FIG. 70. Horizontal sections through the olfactory organs of Polypterus of stages 

 25 (A), 26 (B), and 27 (C). 



c.o, cement organ ; olf, olfactory rudiment ; opt, optic stalk ; Thai, cavity of thalamencephalon. 



hreaks through from the posterior portion of the olfactory organ into 

 the part of the huccal cavity lined by "endoderm." This is a 

 secondary modification of a type which will be discussed in the next 

 chapter in the description of the development of the huccal cavity in 

 these forms. 



The first rudiment of the olfactory organ has been described as a 

 thickening of the ectoderm. As in the case of other nervous or 

 glandular developments of the ectoderm the superficial layer (Fig. 70) 

 takes no part in its formation. Commonly it degenerates and 

 disappears over the actual olfactory epithelium. Again, as frequently 

 happens in the development of primitively hollow organs, the rudi- 

 ment may be for a time solid, forming a simple solid downgrowth in 



