THE ECTODERM AND ITS DERIVATIVES 



Cranial nerve i 



191 



Cranial nerve II 

 Optic chiosmo 



Pituitary 

 gland 



Ventral Dorsal 



Brain of the adult frog. 



Lateral 



Vagus (X) nerve — arises with IX, sending branches to second, third, and 



fourth branchial arches; to lateral line organs and to viscera. 

 Cranial Crest Segments — visceral cartilages, parts of the cranial cartilages 



from head mesenchyme originally derived from ectoderm. 

 Spinal Cord — from rhombencephalon into tail, with continuous cavity. 

 Neural (central) canal — original neurocoel, constricted centrally. 

 Ependymal layer — elongate, ciliated cells lining the canal. 

 Mantle layer — gray matter consisting of compact cell bodies, lateral. 

 Marginal layer — white matter, consisting of outermost axons. 

 Dorsal root — afferent fibers passing dorso-mesially to join dorsal root 



ganglion. 

 Dorsal root ganglion — paired thick bundle of neurons dorso-lateral to 



spinal cord, derived from neural crests. 

 Ventral root — efferent fibers passing ventro-laterally from spinal cord to 



spinal nerve trunk. 

 Spinal nerve — fusion of afferent and efferent fibers associated with distal 



organs (muscles, etc.). 

 Dorsal ramus — branch of spinal nerve to dorsal muscles. 

 Ventral ramus — branch of spinal nerve to ventral muscles. 

 Communicating ramus — fibers from spinal nerve to sympathetic ganglion. 

 Sympathetic ganglion — nerve cells lateral to dorsal aorta, derived from 



neural crest. 

 Special Sense Organs — found in the head region. 



Eye — well developed in 11 mm. stage, some parts mesodermal. 



Retinal layer — sensory portion consisting of rods and cones (inner and 



outer granular layers). 

 Pigmented layer — thinner layer outside of retinal layer. 

 Lens — arising as vesicle, now a solid ball lying in opening of optic cup. 



