A.!. 



ep. 



A.5. 



5^6 EMBRYOLOGY : FROG 



neural folds grow taller, approaching one another and deepening 

 the neural groove, bend over, and meet so as to enclose a space, 



the neural canal, uniting 

 ■■^' first about the middle of 



their length (Figs. 493, 495 A, 

 496 A) . Since they enclose the 

 blastopore vestige, the latter 

 comes to lead from the gut 

 to the neural canal and gives 

 rise to a neurenteric canal, 

 but this soon disappears. 

 The neural canal separates 

 from the ectoderm above it, 

 formed by the outer sides of 

 the neural folds, whose inner 

 sides become the wall of the 

 neural canal. Before the 

 folds have united in front, 

 the open canal between them 

 is divided into three swell- 

 ings, the rudiments of the 

 fore-, mid-, and hind-brain. 

 It will be seen that in the 

 frog, as in the lancelet, the 

 central nervous system arises 

 by the sinking in and folding 

 of a strip of the epidermis of 

 the back. This process is 

 found in all Chordata, and 

 is of the highest importance 

 in the drawing of compar- 

 isons between them and 

 other animals. During the 

 formation of the central 

 nervous system the body 

 has been elongating and 

 other structures appearing. 

 Below the blastopore, in the 

 area which it occupied before its contraction, there appears, as 

 we have seen, a pit known as the proctodaeum, and an opening 

 piercing through from this to the gut forms the anus. From anus 



B. 



Fig. 496. — Diagrams to illustrate the 

 formation of the central nervous sys- 

 tem of the frog. 



A , The folding off of the neural canal (cf. Figs. 491-94) ; 



ep., future epidermis ; n.c, neural crest, growing out 

 to form a dorsal root ; n.f., neural fold ; n.g., neural 

 groove ; n.p., neural plate. 



B, transverse section of the fore-brain at the hatching 



stage ; f.br., fore-brain ; Is., lens ; op.c, inner wall 

 of optic cup, which will form retina proper ; 

 op.c'., outer wall, which will form pigment layer ; 

 o.st., stalk of optic vesicle. 



