EMBR YONIC DE I 'EL OPMENT. 



125 



tion of the archenteron in the usual way. This retarded 

 growth of the notochord anteriorly indicates that its exten- 

 sion to the tip of the snout is a secondary phenomenon. 

 Ancestrally we are bound to assume it did not extend so 

 far forwards. The forward 

 extension of the notochord 

 is, as noted above, obviously 

 useful to Amphioxus in ren- 

 dering its pointed snout 

 sufficiently resistant to en- 

 able it to burrow in the 

 sand. When it wants to 

 bury itself in the sand, it 

 has not to take pains to dig 

 a hole, but darts in in the 

 fraction of a second. 



The histological differen- 

 tiation of the notochord 

 commences soon after the 

 sides of the chordal groove 



*; ^^ 



Fig. 68 bis. — Embryo of Amphioxus, 

 with eight pairs of somites to show the 

 have come together so as to primary relations of the anterior end of 



the notochord. From above. (After 

 Hatschek.) 



p.c. Prcechordal portion of archen- 

 teron, which becomes converted into the 

 head-cavities, n.p. Neuropore. ch, Noto- 

 chord : over which lies the neural tube. 

 viy. Myoccelomic pouches, ne. Neuren- 



obliterate the lumen. The 

 cells composing the noto- 

 chord are, at the first ap- 

 proximation of the walls of 

 the groove, placed end to 'enc canal , , ^ 



° ^ N.B. — In this and other figures of 



Amphioxus embryos here reproduced 

 after Hatschek, the so-called mesoder- 

 mic pole cells have been omitted in 

 accordance with the observations of 

 Wilson and Lwoff. 



end, but soon begin to inter- 

 lace with one another across 

 the middle line (Fig. 65 F\ 

 and finally each cell comes 

 to occupy the whole width of the notochord (Fig. 66). 

 Meanwhile vacuoles begin to appear in the cells (Fig. 66). 

 The vacuolisation of its component cells is an extremely 



