220 



ELEMENTS OF BIOLOGY 



ing embryonic development some of the cells of the neural crest 

 migrate ventrally and take up permanent position in the viscera 

 and elsewhere to become the ganglia of the sympathetic system. 

 The most conspicuous sympathetic ganglia form a chain in the 

 dorsal wall of the body, just ventral to the vertebral column. Fibres 

 from these ganglia ramify to the various organs and tissues, estab- 



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NEURAL FOLD 

 ■. NEURAL FOLD 



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Fig. 149. — Three stages in the early development of the spinal cord and brain as 

 illustrated by the development of the chick. The egg was opened and the early chick 

 embryos examined under a low power of the microscope. In the first figure on the 

 left the head bud is just forming; in the last figure the three primitive divisions 

 of the brain are indicated. The neural tube, the primordium of the adult spinal cord, 

 has not yet closed posteriorly. A thin line visible along the mid-line in each figure 

 represents the notochord, which disappears later, (After Duval.) 



lishing a most surprisingly complete network of sympathetic fibres 

 associated with glands, walls of blood vessels, and tissues in general. 

 Other fibres grow back, so to speak, to establish connections with 

 the central nervous system. By means of these returning rami or 

 fibres the sympathetic and central systems are in communication so 

 that a disturbance in the central system may affect the sympathetic 

 and the organs which it reaches, and stimuli from the viscera may 



