EMBRYOLOGY 



229 



closes by the union of the cells at its margin, the anus opens in another 

 position. The endoderm near the posterior end of the intestine meets 

 the ectoderm, the two layers fuse and then break open. Figure 183 

 shows the anus as still closed, while in Fig. 184 it is open. The time at 

 which the opening of the anus occurs differs greatl}' in different forms. 



It should be borne in mind that Figs. 183 and 184 are diagrammatic, 

 and do not represent a condition prevailing at any one time in embryonic 

 development. For the sake of compactness, organs have been shown 

 in the same figure in stages which do not occur simultaneously. 



Central Nervous System. — -It has already been pointed out, and 

 shown in Figs. 180 and 181, that the early central nervous system is a 

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



Fig. 185. — Successive stages in the development of the lungs. The esophagus is 

 shown in A, B, and C, but not in D. As the lungs grow the mesoderm is pushed before them 

 and thus comes to invest the adult lungs and to make part of the lung tissue, br, bron- 

 chus; es, esophagus; I, lung; m, mesoderm; tr, trachea. 



tube is wide in the anterior region, where it forms the brain, and narrow 

 posteriorly, where it produces the spinal cord. The thickening and fold- 

 ing of the walls of this tube, especially in the formation of the lobes and 

 cavities of the brain, are too complicated to occupy attention here. 



The Nerves. — The nerves extending from the spinal cord take their 

 origin in part from the neural crests. 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. 180, 

 and in Fig. 163. 



As was pointed out in Chapter VI, the large nerves arising from the 

 spinal cord are connected with the cord by two roots, a dorsal root of 

 sensory fibers and a ventral root of motor fibers. The dorsal root is 

 enlarged to form a ganglion. 



