126 THE ANCESTRY OF VERTEBRATES 



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 thishomo- 

 logization is of great importance in regard to another question, 

 viz: the relation of the Craniate mouth to that of i4/n/7^/ox«s. 

 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 figures, 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 spiraculum of fishes, and the mouth 

 of the latter, according to Van Wyhe, with the praeoral pit 

 of Amphioxus. In a former article (1913) I argued 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, 1 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 : "I 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 



