THE ECTODERM AND ITS DERIVATIVES 



181 



Vagus (X)gangli 



Sensory cilia 



Sensory cell of loterol 



line organ 



Splanchnopleure 



4mnn stage- transverse 

 section 



Lateral line nerve 

 extending into tail Lateral line 

 sense organ 



Vagus (X) nerve and 

 origin of lateral line 

 system 



Tadpole 



Origin of the lateral line sense organ system in the frog larva. 



The Cranial Nerves 



These nerves, as in the case of practically all nerves of the embryo, 

 are developed in pairs. Many of them are both sensory (afferent) and 

 motor (efferent). Each nerve will be dealt with as a separate entity. 



As the neural folds close they leave a column of cells between the 

 fold and the dorsal ectoderm. These are known as the cranial or 

 neural crests, depending upon the level under consideration. The cra- 

 nial crests of the brain level are different from the neural crests of the 

 spinal cord level in that they become divided into four large pairs 

 known as the cranial placodes. These consist of thickened patches of 

 neural ectoderm of the head, one of which, the auditory placode, has 

 been described already. The original cellular connection between each 

 cranial crest segment and the brain remains as the sheath of the nerve 

 fibers to be developed in that region. At the spinal cord level the seg- 

 ments are metameric and will be as abundant as the spinal nerves de- 

 veloped therefrom. 



Four ganglionic masses are found in the development of the frog 

 cranial nerves as early as the 6 mm. stage. These include the first 

 or semilunar ganglion of the fifth nerve. The second is the acustico- 

 facialis (VII-VIII) ganglion which becomes associated with the 

 auditory placode. The third is the glossopharyngeal (IX) and the 

 fourth is the vagus (X). The third and fourth arise together as the post- 

 otic ganglionic masses and each becomes associated with an epi- 

 branchial placode. 



