THE FORMATION OF THE LAYERS IN AMPHIOXUS. 303 
collar-cavities still in open communication Avith. the gut, and 
the beginning of the notochord is indicated between them, 
showing that the formation of this structure has also pro- 
ceeded in an anterior direction. In fig. 12 b on the left side 
the collar-cavity is seen to be closed off from the cavity of 
the archenteron, and lying beside it nearer the middle line 
is the trunk-cavity equally shut off from the archentei’on. 
In fig. 12 c the dying out of the collar-cavity can be seen on 
the left side, whilst on the right side the collar-cavity can 
be seen to be closed off from the archenteron. Finally in 
fig. 12 tZ, which is taken from considerably further back in 
the series, the hinder portions of the trunk-cavities or trunk- 
folds can be seen opening freely into the archenteron. It 
follows that the anterior portions of the collar-cavities and 
the ])osterior portions of the trunk-cavities retain their 
openings into the archenteron. The trunk-cavity has already 
had two somites cut off from it. It will be noticed that the 
formation of the notochord is most advanced in those sections 
in Avhich the collar-cavities are visible, recalling the con- 
ditions found in Bal anoglo ss us. 
d'he first indication of the head-cavities is seen when 
five somites have been cut off from the anterior part of the 
trunk-cavity. A section through the front part of such a 
larva is shown in fig. 13. The notochord (c/t.), which by this 
time is completely detached from the archenteric wall, has 
grown forward towards the extreme front end of the embryo. 
Above it we see the nerve-plate grooved and partly covered 
by an ectodermic fold, leaving, however, a pore — the neuro- 
pore — which persists for a long time in the free-swimming 
larva. The archenteron may be seen to be produced above into 
a thin bi-lobed vesicle. This is the rudiment of the tAvo head- 
cavities, and may be compared to the anterior outgrowth Avhich 
gives rise to the proboscis cavity in Balanoglossus. 
Already the left lobe may be seen to have a thicker Avail than 
that on the right side. In fig. 14 a section through an older 
larva in the same region is shown. In this embryo six 
somites have been cut off from the front end of the trunk 
