58 
bestimmen, da eine Abgrenzung des Gehirnes vom Rücken- 
mark auf Schwierigkeiten stösst (4t® 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. 215). 
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 cônsidered 
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 (19,3, 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 
presert 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. — Aîterwards, 
however, in reëxamining 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 à 
certain way proved to be the very missing link between 
Annelids and Craniates which, if it did not exist, 1 shoul 
have had to construct. For, if we assume that Craniates are 
to be derived from Annelid-like ancestors by the con version 
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 
