THE NERVOUS SYSTEM. 



443 



dally. The stage of partial closure of the neural tube is shown in Eternod's 

 figure of a human embryo of 2.1 mm. (Fig. 375, b). This order of closure in- 

 dicates, to some extent, the order of subsequent histological development; the 

 extreme caudal and cephalic extremities are more backward than the parts 

 which close first. The last point to close anteriorly marks, as stated previously 

 (p. 42 1), the cephalic extremity of the neural tube and is the anterior neuropore. 

 As indicated in Eternod's embryo, the anterior end of the neural plate is broader 

 even before its closure; thus when the tube is completed its anterior end is more 

 expanded. This expansion is the future brain, the narrower caudal portion 



Yolk sac 



Amnion 



Neural groove 



FlG. 372. Dorsal view of human embryo, two millimeters in length, with yolk 



von Spee, Kollmann. 

 The amnion is opened dorsally. 



being the future spinal cord. Before the closure of the brain part of the tube 

 the beginnings of the three primary brain vesicles are also indicated (Fig. 84). 

 At this stage the neural plate shows no differentiation into nervous and sup- 

 porting elements. The neural tube is composed of the two lateral walls and 

 the median roof and floor plates (comp. p. 423) (Figs. 307 and 404). 



The appearance of the anterior end of the neural tube with the closure com- 

 pleted, except the anterior and posterior neuropores, is shown in the model of 

 one half of the tube. The external appearance and also the inner surfaces are 

 shown in Figs. 376 and 377. At this stage the cephalic flexure (see p. 424) is 

 already quite pronounced, the cephalic end of the brain tube being bent ven- 



