174 



GENERAL ZOOLOGY 



of a portion of the ectoderm covering the dorsal side of the 

 embryo which is called the neural plate. In most vertebrates 

 the first step in the conversion of the neural plate into a neural 

 tube consists in the elevation of the lateral boundaries of the 

 neural plate into approximately parallel folds or ridges. The 

 crests of these ridges grow toward each other, meet, and fuse 

 in the mid-line, thus forming a tube which soon separates from 

 the under side of the reunited ectoderm (Fig. 114). A slight 

 variation of this occurs in some of the lower vertebrates (cyclo- 

 stomes, teleosts, and ganoids) in that the neural tube is derived 



D E F 



Fig. 114. — Three stages in the development of the nervous system of the 

 salamander, Ambystoma. A, dorsal view of the embryo with open neural folds; 

 B, edges of neural folds meeting in the mid-line; C, neural tube completely formed; 

 D, cross section of open neural folds; E, cross section of closing neural folds; F, 

 cross section of spinal cord of C, showing the cord as a tube pinched off from the 

 ectoderm which has reunited above, fb, forebrain; hb, hindbrain; mb, mid- 

 brain; sc, spinal cord. 



from a thickened keel formed in the mid-line of the neural plate on 

 the under side. This keel acquires a lumen and becomes sepa- 

 rated from the ectoderm above. In all cases, then, a neural tube 

 is formed in vertebrates, and this neural tube develops into the 

 brain and spinal cord. The neural tube also gives rise to all the 

 neurons of the body except those of the olfactory nerve and some 

 elements of the ganglia of cranial nerves V, VII, VIII, IX and X, 

 that come from the ectoderm. 



The walls of the anterior end of the tube thicken and differ- 

 entiate into the embryonic fore-, mid-, and hindbrain, while the 

 posterior portion of the tube becomes the spinal cord. The 

 cavity of the neural tube in the region of the brain is represented 

 by the ventricles and in the spinal cord by the central canal. 

 Both ventricles and spinal canal are filled with cerebrospinal 



