THE CEPHALOCORDA 



193 



from the neural tube to the archenteron. This passage, 

 known as the neurenteric canal, is present in the embryos 



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

 JV, 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 ; K, horizuntal 

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

 5", horizontal section of an embryo, with fourteen pairs of mesoblastic s< mites, 

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

 six hours old, showing the mouih, first gill-slli, etc. ack, archenteron ; 

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

 h.cl, left head cavity ; /^r?-, right head cavity ; in, mouth ; j7zes, mesoblasticsomites; 

 wr, neural canal ; nc/i, notochord ; nee, neurenteric canal ; np, neuropore. I-IX, 

 the several pairs of mesoblastic somites. 



of higher vertebrates. As the neural plate is enclosed by 



II. N 



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