6:2 AMPHIOXUS. 



gill-slit are formed. The embryo elongates very rapidly, and 

 becoming much narrower and more slender, gradually acquires 

 a shape and proportions resembling those of the adult. During 

 the whole period the embryo is pelagic : swimming is effected 

 at first by the flagella clothing the surface, but towards the close 

 of the period the muscles of the body-walls become definitely 

 established, and the young Amphioxus swims by means of 

 muscular contractions, like the adult. 



Although there is a great increase in length during the 

 period, there is little if any change in bulk, and it is doubtful 

 whether the embryo obtains any food from without until the 

 formation of the mouth at the close of the period. 



Fig. 27.— Amphioxus embryo shortly after hatching, with five pairs of meso- 

 blastic somites; seen in optical- section from the right side, x 22i 

 (After Hatschek.) 



E, epiblast. H, hypoblast. NC, neural canal. UF, neural fold. WK, neurcpore. 

 UT, neurenteric canal. PC, polar mesoblast cell. SI, first mesoblastic somite of right 

 side. T, archenteron. 



In dealing with this period in the developmental history it 

 will be convenient to describe the several systems one by one. 



1. The Nervous System. 



After hatching of the embryo, the closure of the neural canal 

 by fusion of the neural folds, proceeds rapidly forwards (Pigs. 

 25 and 27), and soon reaches the anterior border of the first somite 

 beyond which level the nervous system does not extend. 



From the mode of its formation (Figs. 23, 24, 26, and 27), the 

 neural canal is, in its early stages, merely the space between the 

 neural plate and the overlapping lateral plates of epiblast and 

 has at first no independent roof of its own. The canal is at 

 first wide from side to side, but shallow dorso-ventrally. 



