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PRINCIPLES OF ANIMAL BIOLOGY 



Nervous System. — It has already been pointed out, and shown in 

 Figs. 182 and 183, that the early central nervous system is a tube formed 

 by the fusion of two folds or ridges of the ectoderm. This tube is wide 

 in the anterior region, where it 

 forms the brain, and narrow pos- 

 teriorly, where it produces the 

 spinal cord. The thickening and 

 folding of the walls of this tube, 



Fig. 185. — Diagram representing the development of .some of the organs of vertebrate 

 animals, early and later stages. The figures are a trifle to one side of the median plane. 

 The stages shown are not necessarily contemporaneous, an, anus; bd, bile duct; hr, brain; 

 c, coelom; dp, dorsal rudiment of pancreas; gh, gall bladder; int, intestine; Ig, lung; li, liver; 

 m, mouth; ms, mesoderm; nd, notochord; pc, pericardial chamber; rt, root of tongue; sp, 

 spinal cord; st, stomach; t, tongue; vp, ventral rudiment of pancreas. 



especially in the formation of the lobes and cavities of the brain, are 

 very complicated processes. 



The nerves extending from the spinal cord take their origin in part 

 from the neural creds. These crests are masses of cells budded off from 

 the inner surface of the ectoderm at or near the region of the neural folds, 

 as indicated in the cross section of the frog (Fig. 182, nc) and in Fig. 17(5. 



As was pointed out in Chap. 13, the large nerves arising from the 

 spinal cord are connected with the cord by two roots. The dorsal root is 

 composed of afferent fibers and the ventral root of efferent fibers. The 

 dorsal root is enlarged to form a ganglion. 



The dorsal ganglion is in each nerve developed from one of the neural 



