OHAPTKI,' XVII 



THE NERVOUS SYSTEM 



THE DEVELOPMENT OF THE NERVOUS SYSTEM 



Am. 



The first external evidence of the beginning of the nervous system is 

 the appearance of a median longitudinal furrow in the dorsal (neural) 

 ectoderm of the very young embryo 

 (about the fifteenth day in man). 

 This is known as the neural or 

 medullary groove. Still earlier 

 stages, as revealed in sections, in- 

 clude first an increase in height of 

 the ectodermal cells to form an axial 

 plate of cuboidal cells, the neural 

 plate; and secondly the conversion of 

 this simple layered plate into a struc- 

 ture of several strata of cuboidal 

 cells, which structure meanwhile 

 forms the floor and lateral walls of 

 the developing neural groove. The 

 neural plate apparently grows more 

 rapidly laterally causing thus a pro- 

 gressive elevation of its borders, the 

 neural folds, and producing in con- 

 sequence a gradually deepening neu- 

 ral groove (Fig. 506). According to 

 Glaser (Anat. Rec., 8, 12, 1914), the 

 inequality in growth between neural 

 plate and the adjacent ectoderm 

 which causes folding is due to great- 

 er water absorption on the part of 

 the plate. By continuation of this 

 process of unequal growth medially 

 and laterally, the folds finally meet 



in the dorsal mid-line and fuse to form the neural tube. On both sides of 

 the line of fusion a group of ectodermal cells appears, the neural or gan- 

 glionic crest, from which arise the neurons of the dorsal and sympathetic 



587 



FIG. 505. HUMAN EMBRYO 2 MILLI- 

 METERS LONG. (Graf Spee.) X 30. 



Am, amnion; C, chorion; C.V., chori- 

 onic villi; Bs, body stalk (embryo- 

 phore) ; Mg, medullary groove; Nc, neu- 

 renteric canal; P.s, primitive streak; 

 Ys, yolk-sac. (From Williams.) 



