THE FORMATION OF THE LAYERS JN AMFHIOXUS. 305 
The latei’ history of tliese coelomic vesicles aud of the rest 
of the archenteron which constitutes the definitive gut may 
be followed in larvae, in which the mouth is about to be 
formed aud the first gill-slit about to break through. This 
stage is almost the latest to which it is possible to rear the 
eggs artificially ; for older larvae one has to depend on the 
tow-net. A series of seven sections through such an embryo 
is represented in figs. 16 a-g, fig. 16 a being, of course, the 
most anterior section. In this section the neuropore is seen 
and the notochord lying beneath the nerve-cord. At the 
sides of the notochord the slit-like anterior ends of the collar- 
cavities can be seen, whilst beneath the notochord is seen the 
thin-walled right head-cavity which has become shifted 
anteriorly to its left fellow. This is one of the first signs of 
that curious asymmetry which is one of the peculiarities of 
Amphioxus larvae, and which has so puzzled all investigators. 
A satisfactory, or at any rate, a plausible explanation of it 
has been suggested to lue and will be given later on. In the 
more posterior section tig. 16 h, the left head-cavity is seen 
lying above the right one as a completely closed thick- walled 
vesicle. A vacuolated ectodermal cell (r.) marks the spot 
where this vesicle will later acquire an opening to the 
exterior. 'L’his section therefore completely refutes the view 
of Legros (23) that this vesicle is an ectodermal invagination. 
Van ^Vijhe (35) had already decided in my favour after 
exiiminiug preparations sent him by both Legros and myself. 
Tlie nerve-tube is closed and at the sides of the notochord 
the collar-cavities are seen as spacious cavities, the inner 
walls of which are, however, becoming thickened. In fig. 16 c 
the anterior blind end of the gut is grazed, and in the 
thickened inner walls of the collai-caviiies adjacent to the 
notochord numerous muscular fibrils {muse.) are seen — these 
being the first traces of the formation of the first myotome. 
It is of particular importance to note that the lower parts of 
the collar-cavities which are wedged in between the ectoderm 
and the gut are in open communication with the upper 
myotomic portions. Tlie cavity on the right side extends 
