Integrative Systems 



143 



SPINAL CORD 

 DERMATOME 

 SOMATIC MOTOR NERVE. 

 MYOTOME 



PRIMITIVE DUCT. 

 COELOM 



SENSORY GANGUON 



NOTOCHORD 



HYPOCHORDA 

 "DORSAL AORTA 



POSTCARDINAL VEIN 

 SPINAL NERVE 



Fig. 134. A cross section of a 17 mm. elasmobranch (Squalus) embryo, in the 

 trunk region, showing an early stage in the formation of sympathetic ganglions. 

 The yolk-sac to which the embryo is attached has been removed. (Courtesy, Neal 

 and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



shaped into a tube which extends through the length of the embryo 

 (Figs. 225, 228). This neural tube is immediately dorsal to the noto- 

 chord. The wider anterior region of the tube enlarges to form the highly 

 complex brain, which retains its continuity with the longer, narrower, 

 and less complex remainder of the tube, the spinal cord. 



From the wall of the neural tube, cells, or groups of cells, which 

 are prospective neurons become detached and migrate to deeper posi- 

 tions in the embryo, where they become massed and develop into 

 ganglions which, in the adult, lie in various regions external to the 

 brain and cord, even in quite deep parts of the body (Fig. 134). From 

 cells in these ganglions, nerve-fibers may grow back into the central 

 nervous tube. From cells in the wall of the tube, nerve-fibers grow 

 outward and become collected into bundles, nerves, which extend into 

 all, even the most remote, parts of the body. So far as is known, all 

 ganglions and nerves external to the brain and cord are derived from 

 the neural tube, or from ectodermal thickenings (placodes) closely 

 adjacent to it, with the single exception of the nerves from the nasal 

 organ of smell. The fibers of these olfactory nerves grow out from 

 the olfactory receptor ceils in the lining of the nasal cavities and pass 

 into the front end of the brain. 



In the adult the brain and spinal cord are dorsal to the alimentary 

 tube and also to the notochord. They are hollow, retaining the central 



