THE ANCESTRY OF VERTEBRATES- 



mandibular segment, but this was denied by VAN Wyhe 

 (1882, p. 8) and recently by ZlEGLER (1908, p. 659). 

 Gegenbaur {1887, p. 7) suggests that nevertheless in phy- 

 logeny the praemandibular segment may have split off 

 from the mandibular segment. In Necturus Miss PlatT" 

 (1897, p. 443) finds that the praemandibular mesoderm m 

 young stages is not divided from the mandibular. In 

 Petromyzon recently Hatschek (1910, p. 481) states that 

 here also the praemandibular and mandibular cavities 

 originally represent one segment, the mandibular segment,, 

 which accordingly is the first mesoderm segment, to which' 

 a special significance is attributed by HATSCHEK. It is the 

 mesoderm of his "acromerite," an anterior part of the body 

 corresponding to my prostomium + mandibular segment, 

 and concerning which HATSCHEK provisionally leaves it 

 undecided, as to whether it represents an anterior unseg- 

 mented region or an anterior, somewhat diverging, metamere. 

 HATSCHEK thinks "dass die Vorgange bei den Selachiern 

 erst durch die in mancher Beziehung einfacheren und wohli 

 auch primitiveren bei Petromyzon besser verstandlich wer- 

 den," and thus seems to be inclined to join GegENBAUR's^ 

 opinion. In Amphioxus also we see the first pair of segments,, 

 homologized by VAN WYHE to the mandibular segments 

 of Craniates, send out each a forward prolongation into the' 

 prostomium. 



That the praemandibular cavities produce muscles {Muse, 

 rectus superior, internus and inferior, and Muse, obliquus 

 inferior, as stated in Selachians), and in this respect resemble 

 myotomes of the trunk, cannot well be adduced as an 

 argument for considering them as representing a segment,, 

 since in the first place it may be doubted if the eye-muscles 

 are to be derived directly from segmental muscles. We 

 see the latter vanish gradually in going from the trunt 

 to the head in an embryo. The three occipital segments in" 

 Pristiurus and Seyllium s{\\\ have a distinct myotome with 

 muscle-formation, the first vagus-somite has only a rudi- 

 mentary myomere, while in the segments lying in front of this,, 

 the glossopharyngeus- and the facialis-segment, no myotoms 

 are developed. In the (hyoid-?) mandibular- and praeman- 

 dibular segment we then see the eye-muscles appear, whicb 

 accordingly are considered, e.g. by ZlEGLER (1908, p. 674), 

 as relatively young muscles, not directly comparable to- 

 segmental trunk muscles. But even if we do not share 



