186 



FOUNDATIONS OF BIOLOGY 



/ 



the nervous system is a longitudinal groove in the ectoderm 

 along the dorsal surface, which soon becomes converted into 



a tube by the apposition and, 

 finally, the fusion of its edges. 

 This NEURAL TUBE then becomes 

 separated from and sinks below 

 the surface ectoderm, and in time 

 forms the CENTRAL nervous sys- 

 tem consisting of the brain and 

 spinal cord. As development pro- 

 ceeds, outgrowths from the central 

 nervous system establish the 

 PERIPHERAL and the AUTONOMIC 

 (SYMPATHETIC) nervous systems, 

 so that structurally as well as 

 physiologically the whole nervous 

 system represents a unit; a single 

 organ, as it were, which seconda- 

 rily becomes closely identified 

 here and there with sense organ, 

 muscle, or gland, as the case 

 may be. 



The first marked structural 



FIG. 103. Diagram of a section 

 (highly magnified) of the wall of 

 the intestine of a Vertebrate to 

 show its intrinsic nervous organiza- 

 tion which brings about the move- 

 ments of the tube. The two plexuses 

 consistessentially of simple neurons modifications in the developing 



central nervous system of Verte- 

 brates are two constrictions of the 

 enlarged anterior end of the neural 

 tube, which delineate the three 

 primary brain vesicles: FORE- 

 BRAIN, MID-BRAIN, and HIND-BRAIN. Thus very early in 

 embryonic development, one end of the neural tube is 

 molded into the brain, leaving the rest to form the spinal 

 cord. (Fig. 104.) 



arranged as nerve nets. a, food 

 absorbing surface of the intestine; 

 b, mucous layer; c, plexus of neu- 

 rons (submucous) ; d, circular mus- 

 cle; e, plexus of neurons (my en- 

 teric); /, longitudinal muscle; g, 

 serous layer. (From Parker, after 

 Lewis.) 



