406 NORMAL HISTOLOGY. 



The olfactory plates are converted into the nasal pits by the 

 growth and elevation of the surrounding parts on all sides except 

 the under surface, along which the nasal pits for a time directly com- 

 municate with the primary mouth. In addition to the differentiation 

 of the surrounding tissues into the structures of the external nose, 

 the close relation of the primary nasal surface with the brain-vesicle 

 disappears with the changes of position produced by the develop- 

 ment of the fore-brain and growth of the tissues forming the cranial 

 case, particularly the development of the olfactory ganglion from 

 the olfactory plate. 



The complicated surfaces of the nasal fossae are due primarily to the 

 appearance of the superior, middle, and inferior turbinal folds on 

 the lateral wall of the nasal recess. Each fold comprises the dupli- 

 cation of ectoderm enclosing a core of mesoderm ; the latter 

 becomes the turbinal cartilages and finally the" corresponding 

 osseous plates. The differentiation of the olfactory region from 

 the general lining of the nasal fossae takes place coincidently with the 

 growth of the olfactory nerve-fibres ; the details of the histogenesis 

 of these structures, however, are still but imperfectly determined. 



The special organs of taste and of touch, including the taste- 

 buds and the tactile corpuscles, have been already considered in 

 connection respectively with the tongue and with the peripheral 

 nerve-endings and the skin. 



