THE CEPHALOCORDA 193 



from the neural tube to the archenteron. This passage, 

 known as the neurenteric canal, is present in the embryos 



nee 



Fig. 48 



N to T. Further stages in the development of Amphioxus (after Hatschek). 

 TV, sagittal section of an embryo with three mesoblastic somites ; O, horizontal 

 section of the same embryo ; P, sagittal section of an older embryo ; Q, sagittal 

 section of an embryo with nine pairs of mesoblastic somites ; R, horizontal 

 section of an embryo of the same age showing the origin of the head-cavities ; 

 S, horizontal section of an embryo, with fourteen pairs of mesoblastic Semites, 

 showing the position of the head-cavities at this stage ; T, a larva about thirty- 

 six hours old, showing the mouth, first gill-slit, etc. ach, archenteron ; 

 an, anus ; cf. caudal fin ; c.gl, club-shaped gland ; e, eyespot ; gs^, first gill-slit ; 

 h.cl, left head cavity ; her, right head cavity ; ;;/, mouth ; mes, mesoblastic somites ; 

 we, neural canal ; nc/t, notochord ; nee, neurenteric canal ; n/>. neuropore. I-IX, 

 the several pairs of mesoblastic somites. 



of higher vertebrates. As the neural plate is enclosed by 



II. N 



