the mandibular cavities of Craniates to this first pair of 
normally developed somites in Amphioxus is supported by 
VAN WYHE (1907, p. 75) by the argument that the endostyle 
of Amphioxus-and: the thyreoid-gland in Craniates both 
originate from the ventral wall of the gut under this segment. 
The mouth of Amphioxus. — The question of this homo- 
logization is of great importance in regard to another question, 
viz: the relation of the Craniate mouth to that of Amphioxus. 
The mouth of Craniates is situated as a median structure 
in front of the mandibular segment, that of Amphioxus 
originates as a structure belonging to the left side, as stated 
bij VAN WYHE (1914, p. 64) and as may be seen also from 
HATSCHEK’s figuies, and behind the mandibular segment. 
Afterwards the left mandibular cavity grows out as a ring 
round the mouth. Thus the mouth of Amphioxus must be 
compared with the left spiraculuni of fishes, and the mouth 
of the latter, according to VAN WYHE, with the praeoral pit 
of Amphioxus. In a former article (1913) largued that VAN 
WYHE's mandibular segment of Amphioxus is to be consi- 
dered not as the second but as the first segment of the 
soma. Viewing with VAN WYHE the praemandibular segment 
as the first segment of Craniates, l was thus led to homologize 
the latter to VAN WYHE's mandibular segment in Amphio- 
xus. If this view were right, the mouth of Amphioxus 
would not correspond to the spiracle of fishes but to the 
left half of the mouth. NEAL (1898, p. 267) was led to a 
similar conclusion: “Il regard the mouth of Amphioxus as 
homologous with the left half of the mouth of Craniata and 
the club-shaped gland as its antimere. That the mouth of 
Amphioxus as an organ of the left side is exactly homologous 
with the left half of the mouth of Squalus appears to me 
probable on the a priori ground that it is improbable that 
an organ of the same function should be twice acquired 
in the Vertebrate series”. 
However, we have seen that arguments may be adduced 
as well for another view, viz: that BALFOUR's praeman- 
dibular cavities do not represent a segment and that 
the first segment in Craniates is that of the mandibula. 
Then VAN WYHE’s homologization would prove to be the 
right one with this restriction, that in Amphioxus as well 
as in Craniates we should have to do not with the second 
but with the first body segment. The comparison also of 
the mouth of Amphioxus with the left spiracle of Craniates 
