EMBR YONIC DE VEL OPMENT. 



125 



fl.C 



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 wi ,f * 



have Come together SO as tO primary relations of the anterior end of 



the notochord. From above. (After 



obliterate the lumen. 1 he HATSCHEK.) 



cells composing the noto- ** ? s ? c i rdal portion J ar t cl T 



teron, which becomes converted into the 



Chord are, at the first ap- head-cavities. ./. Neuropore. ch. Noto- 



. chord; over which lies the neural tube. 

 proximation Of the walls OI my _ Myocoelomic pouches, ne. Neuren- 



the groove, placed end to teri ^ T c * naL T 



N.B. In this and other figures of 



did, but SOOn begin tO inter- Amphioxus embryos here reproduced 

 . after Hatschek, the so-called mesoder- 



lace with one another across mic pole cells have been emitted in 



the middle line (Fig. 65 F\ accordance with the observations of 



WILSON and LWOFF. 

 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 



