11 • THE NERVOUS SYSTEM 



w. 



ith some understanding of the principles involved in the develop- 

 ment of the nervous system, we may now go on to a study of the individual 

 parts of this system in the chick embryo. We have seen that the nervous system 

 originates during gastrulation and the formation of the primitive streak. At 

 this time the notochord induces the neural plate to differentiate from the rest 

 of the ectoderm. The plate then folds to form the neural tube (Fig. 94, A 

 and B) , and this folding is caused by a contraction of the upper surface of the 

 plate, since isolated portions of the plate will also form neural tubes. As the 

 edges of the neural folds fuse, two strips of tissue bud off from either side 

 and form the neural crests (Fig. 94, C) . The neural crests are versatile tissues, 

 since they form not only the neuroblasts of the spinal ganglia but also the 

 sympathetic ganglia, the sheath cells which are associated with the deposition 

 of myelin sheaths upon nerve fibers, the cortex of the adrenal gland, the 

 pigment cells of the feathers, and some mesenchyme. 



Neural crests and their derivatives 



The performance of these varied tasks by the neural-crest cells necessi- 

 tates considerable migration. This is illustrated in Figure 94, C, by broken 

 lines and arrows. Some crest cells migrate to the region of the dorsal aorta 



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