124 THE ANCESTRY OF VERTEBRATES 



bestimmen, da eine Abgrenzung des Gehirnes vom Riicken- 

 mark auf Schwierigkeiten stosst (4^^^ ed. 1898, p. 171). 



While KUPFFER (1906, p. 14) and HATSCHEK (1892, p. 138) 

 who also homologizes the brain of Craniates to that of 

 Amphioxus cannot see any relation between the pigment 

 spot of the latter and the eyes of the former, others, like AYERS 

 (1890), try to derive the one from the other (cf. p. 29). 



As shown by all this, the points of divergence among 

 the authors are numerous. 



It cannot be denied that in several respects Amphioxus 

 shows very primitive features, so much so, that in text-books 

 of comparative anatomy as well as of embryology, it is very 

 generally taken as a starting point in exposing the structure 

 and development of Vertebrates. It has even been argued 

 that, if Amphioxus did not exist, it ought to be invented. 

 On the other hand those who considered the Annelids as the 

 ancestors of Vertebrates have always been embarrassed 

 more or less by the difficulty of fitting Amphioxus and 

 the Ascidians into their system, and generally considered 

 them, with DOHRN (1875, p. 32), as a degenerated branch 

 of the Vertebrate phylum. This view, however, also fails 

 to remove all the difficulties and in the first exposition of 

 my theory (1913, p. 705) 1 did not see any solution other 

 than to assume that the Acrania have had a quite separate 

 origin, and are perhaps to be derived from other Proto- 

 stomia in a similar way as the Vertebrates from the Annelids. 

 None of the sense-organs which according to my theory were 

 inherited by the Vertebrates diiectly from the Annelids are 

 present here, and apparently no such significance as in 

 Craniates can be attributed to the tip of the notochord 

 which reaches far in front of the brain vesicle. For Ascidian 

 larvae and Appendicularians these difficulties are no less. 



Amphioxus in the light of my theory. — Afterwards, 

 however, in reexamining the question (Delsman, 1913), it 

 appeared to me that Amphioxus at least, far from being an 

 obstacle to my theory, fits perfectly well into it, nay, in a 

 certain way proved to be the very missing link between 

 Annelids and Craniates which, if it did not exist, I should 

 have had to construct. For, if we assume that Craniates are 

 to be derived from Annelid-like ancestors by the conversion 

 of the stomodaeum into the neural tube and by the folding 

 in of part of the apical plate which thus forms the brain 

 vesicle, we may expect the existence of an intermediate 



