58 AMPHIOXUS. 



each other in the median plane so as to completely cover over 

 the central plate (Fig. 24). 



The central plate of epiblast, which thus becomes roofed 

 over, is spoken of as the neural plate (Figs. 22-24, NP), and 

 becomes converted, later on, into the central nervous system. 

 By longitudinal folding of the neural plate a groove is formed 



FIG. 23. 



FIGS. 21 to 24. Transverse sections across the bodies of Amphioxus embryos, 

 showing the mode of formation of the nervous system and of the meso- 

 blastic somites, x 350. (After Hatschek.) 



Pig. 21. Transverse section across the middle of the back of an embryo of the same 

 age as those shown in Figs. 19 and 20. Fig. 22. Transverse section across a slightly 

 older embryo, with one pair of mesoblastic somites, and commencing nervous system. 

 Fig. 23. Transverse section across the same embryo as Fig. 22, but taken rather further 

 back, the section passing through the middle of the first pair of somites. Fig. 24. Trans- 

 verse section through an embryo at the time of hatching ( cf. Figs. 23 and 24) : the section 

 passes through the middle of the first pair of mesoblastic somites, and shows also the 

 mode of formation of the neural tube. CE, enterocoel or mesoblastic somite. E, epi- 

 blast. Gr, archenteron. H, hypoblast. WF, neural fold. WP, neural plate. 



along its upper surface, and this groove, when roofed over by the 



lateral plates or neural folds, becomes the neural canal (Fig. 24). 



The neural plate extends back to the blastopore, which, as 



already described, is situated on the dorsal surface of the hinder 



