THE FOEMATION OF THE LAYERS IN AMPHIOXUS. 311 
He supposes that in one stage of its history Amphioxus 
swam in spirals, turning ever from right to left. Under these 
circumstances he supposes that the first gill-slit of the left 
side, corresponding to the left spiracle of Elasmobranchii, 
became enlarged so as to function as a new mouth. He 
identifies the pi’esent mouth with the spiracle, because he 
supposes that the collar-cavity passes down under it and not 
over it, and he quite justly identifies the collar-cavity with 
the mandibular cavity of Elasmobranch embryos. In the 
supposition that the mouth of Amphioxus lies behind the 
lower extension of the collar-cavity he is mistaken, as I hope 
the present paper will convince him. I am sorry to differ 
from Van Wijhe, for whose generous recognition of my results 
I am grateful, but I think that my theory of the twist of 
a pre-existing mouth is preferable to any theory which 
postulates the formation of a new one, and more in line with 
what we know to have occurred in other divisions of the 
animal kingdom. 
We may sum up our general survey of the formation of 
mesoderm and notochord in Amphioxus thus : Both meso- 
derm and notochord in Amphioxus are of endodeemal 
origin, and the mesoderm originates in a manner closely 
recalling that which Bateson described for Balanoglossus; 
the differences being that the proboscis cavity divides into 
two completely separate head-cavities, that the collai--cavities 
have long antero-doi’sal and postero-ventral extensions, ai’e in 
a word obliquely orientated, and that the trunk-cavities 
become bi’oken up into a series of somites, the dorsal portions 
of which form muscular masses, whilst the moi’e ventral por- 
tions form an undivided splanchnocele. The notochord, as in 
lialanoglossus, becomes mai-ked fii'st in the collar region 
(see fig. 12), and grows forward into the head region sub- 
sequently. The hindermost part of the gut divides into 
neurenteric canal and anal diverticulum. 
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