THE DEVELOPMENT OF THE FROG 279 



detailed here. The lumen of the canal is much reduced, 

 but persists as the central canal of the spinal cord. 



The cranial and spinal nerves are formed in close connection 

 with the neural tube. The dorsal roots of the spinal nerves, 

 and the roots of the vagus, auditory, facial, and trigeminal 

 nerves are formed from the deeper or nervous layer of the 

 epiblast at a very early stage before the neural folds have 

 closed in. They are first recognisable as outgrowths of the 

 lateral edges of the neural plate, and after the neural canal 

 has closed they appear to arise from a ridge-like thickening 

 along the dorsal side of the neural tube, known as the neural 

 crest. The ventral roots of the spinal nerves arise later as 

 ventro-lateral outgrowths of the spinal cord which soon be- 

 come connected with the dorsal roots. 



The organs of special sense are formed wholly or in part 

 as paired involutions of the deeper or nervous layer of the 

 epiblast. Their development is rather complicated, and can 

 only be touched upon here. 



The olfactory organs are formed as a pair of pits at the 

 anterior end of the head, in which both layers of the epiblast 

 are involved. The deeper parts of these pits expand to form 

 the olfactory chambers, their walls, are thrown into numerous 

 folds, and their openings persist as the anterior nares. The 

 posterior nares are formed as downgrowths or diverticula of 

 the olfactory chambers, which grow towards and fuse with 

 the roof of the pharynx and presently open into it. 



The auditory organs are formed as a pair of invaginations 

 of the nervous layer of the epiblast at the sides of the hind 

 part of the head. In this case the outer or epidermic layer 

 of the epiblast does not share in the invagination, so the 

 auditory organs never open to the surface. Each invagination 

 becomes a pit, which is converted into a closed vesicle and 

 loses its connection with the epiblast. The vesicles become 

 surrounded by mesoblast, from which the cartilaginous auditory 

 capsules are developed. The walls of the epiblastic vesicles 

 are converted into the membranous labyrinth, and after the 

 metamorphosis the hyomandibular cleft enters into relations 

 with the auditory organ, and forms the middle ear or tympanic 

 cavity and the Eustachian passages. In the case of the organs 

 of sight, only a part of the eye — viz. the lens — is formed from 

 the surface epiblast. The retina and the pigmented coat of 



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