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THE GERM LAYER DERIVATIVES 



2. Cranial nerves I to X. 



3. Spinal nerves. 



4. Other neural crest derivatives: dorsal root ganglia, sympathetic 

 nervous system, chromatophores, parts of the visceral and cranial 

 cartilage, and the medullary portion of the adrenal glands. 



The Organs of Special Sense 

 The Eye. 



The optic vesicles arise early, by the tail-bud stage, as lateral diver- 

 ticula from the ventro-lateral walls of the diocoel. The connection of 



Ectoderm or 

 cornea 



Opticoel 



Pigmented layer 



Sensory layer 

 rods and cones 



Optic stalk 



"Choroid knot 

 Choroid fissure 



Inverted groove for 



optic nerve and blood vessels 



Schematic diagram of the developing eye parts 

 of the frog. 



the brain cavity with the optic vesicles becomes constricted, by the 

 convergence of the surrounding mesenchyme, into a tube known as 

 the optic stalk. 



The dorso-lateral wall of each optic vesicle comes into contact with 

 the head ectoderm, as it expands laterally, and it is then flattened and 

 finally invaginated. This invagination begins ventro-laterally and is 

 continued obliquely mesio-dorsally. This process of invagination is 

 aided by a thickening of the vesicle wall between the dorsal and 

 ventral limits. This is the region which will later form the retina. 

 This thickening begins at the first point of contact (dorsal) and as a 

 result the connecting optic stalk is moved to a more ventral position. 



