NERVOUS SYSTEM OF LARVA 



NERVOUS SYSTEM AND SENSE ORGANS 



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The origin of the central nervous system has already been 

 described. Owing to the shape of the surface of the egg upon 

 which the neural plate is formed the fore-brain is bent sharply 

 downwards round the anterior end of the notochord (Fig. 495 B). 

 This is known as the cranial flexure. The dorsal roots of the nerves 

 are formed as growths from structures known as the neural 

 crests. These are a pair of internal ridges which project from the 

 sides of the neural plate, near the tops of the neural folds, before 

 the latter have met. The parts of the crests which do not become 

 nerve roots are converted into mesenchyme and form, among 

 other things, the visceral skeleton. The ventral roots arise later, 

 as outgrowths from the side of the central nervous system, and 

 those of the spinal cord become connected with the corresponding 

 dorsal roots. The formation of the olfactory organs has been 

 mentioned. The posterior nares arise from the olfactory chambers 

 as downgrowths which break through into the mouth. The 

 labyrinth of the ear is formed from the deeper layer of the ecto- 

 derm as an ingrowth which forms a vesicle, but does not open 

 to the exterior. It gradually takes on the shape of the labyrinth 

 by the formation of septa which grow into it and divide it up. 

 The eye has a more complicated origin. The retina and the pig- 

 mented epithelium arise from a pair of outgrowths of the fore- 

 brain, known as optic vesicles, which grow out towards the sides 

 of the head soon after the closure of the neural tube. P2ach takes 

 on the form of a hollow bulb on a hollow stalk. The stalk gives 

 rise to the optic nerve. The outer half of the bulb becomes 

 thickened and then folded back into the inner half, as a hollow 

 india-rubber ball may be folded when it has been punctured 

 (Fig. 496 B) . The two-layered cup which thus arises is known as 

 the optic cup. The thick layer which hues it is the retina, the thin 

 layer on the side towards the stalk is the pigment layer. Where the 

 stalk enters, the cup is incomplete, the gap being the choroid 

 fissure. The stalk gradually pulls back and the edges of the fissure 

 fuse, so that the stalk appears to pierce the base of the cup. As 

 a result of this arrangement the line of nerve fibres runs from the 

 outer surface of the retina to the inner and then into the optic 

 nerve. From the deeper layer of the epiblast there arises a thicken- 

 ing which projects into the mouth of the cup, separates from the 



