THE CEPHALOCORDA 



193 



from the neural tube to the archenteron. This passage, 

 known as the neurenteric canal, is present in the embryos 



jnes. 



jnes. 



JVC 



Jich 



Fig. 48 



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

 IV, 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 ; 7?, horizontal 

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

 6", horizontal section of an embryo, With fourteen pairs of mesoblastic somites, 

 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. ack, archenteron ; 

 a, anus ; cf, caudal fin ; c.gt, club-shaped gland ; , eyespot ; g-s 1 , first gill-slit ; 

 h.cl, left head cavity ; her, right head cavity ; m, mouth ; mes, mesoblasticsomites ; 

 nc, neural canal ; ncA, notochord ; nee, neurenteric canal ; /, neuropore. /-/.A", 

 the several pairs of mesoblastic somites. 



of higher vertebrates. As the neural plate is enclosed by 



II. N 



