538 .THE HUMAN EMBRYO. 



petally, from the ganglia into the brain or cord : the ganglia 

 themselves, though developed in close relation with the brain and 

 cord, are not really parts of these, but are independent structures. 

 The formation of neuroblasts in the olfactory epithelium presents 

 no difficulty, when it is remembered that the wall of the brain 

 or spinal cord is itself merely a specialised portion of the surface 

 epithelium ; while, finally, it has been shown in earlier parts 

 of this book that in other Vertebrates, such as the frog and 

 chick for example, the surface epithelium may, in the hinder 

 cranial nerves, take a direct share in the formation of the 

 nerve ganglia. 



It is probable indeed that the mode of development of the 

 olfactory nerve described above, as observed in human embryos, 

 represents a more primitive type of nerve development, from 

 which that of the other sensory nerves has been derived. 



4. The Sympathetic Nervous System. 



The mode of development of the sympathetic nervous system 

 has not been accurately determined in human embryos. From 

 the close resemblance in the mode of development of other parts 

 of the nervous system, it is probable that it takes place in essen- 

 tially the same manner as described above in the case of the 

 chick and rabbit. 



THE DEVELOPMENT OF THE SENSE ORGANS. 



1 . The Nose. 



All the essential points in the development of the olfactory 

 organ have been already described. The formation of the 

 olfactory pit, the organ of Jacobson, the external nostrils, the 

 bridge and alee of the nose, and the posterior narial passages, 

 are dealt with on pp. 494 to 498 ; and the development of the 

 olfactory lobe and olfactory nerve on pp. 517 and 537. 



During the third month the olfactory pits, which are at first 

 simple, become greatly complicated by folding of their walls ; 

 and in this way the nasal labyrinth, supported by the turbinal 

 scrolls, is established. The accessory cavities communicating 

 with the nose, i.e. the antrum, and the frontal, sphenoidal, and 

 ethmoidal sinuses, are not established until a later stage. 



