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difficulties connected with it. I was not thinking at all of 
the problem of the origin of Vertebrates nor did | expect 
ever to occupy myself with it by proposing a new theory 
when, in the course of embryological researches on Inver- 
tebrates, an idea occurred to me which, when | tried to 
work it out, scon led me in the direction of the theory 
of the Annelidan origin of Vertebrates. The combination 
of both proved to be of unexpected fruitfulness and to open 
up new perspectives in many directions. And not only did all 
the difficulties which had proved unsurmountable for nearly 
half a century appear to be solved at once in a most 
unexpected way, but as many new arguments in favour 
of the Annelidan theory were provided by the elaboration 
of this idea. 
After the first publication in 1913(a), further reflections 
and investigations have yielded more than one valuable con- 
firmation of the results reached therein, and sometimes also 
have led to certain modifications, completions and correc- 
tions of my original views. Since these later results were 
published in a number of short articles which are not always 
easily consulted by everybody, it has for some time been 
my intention to unite them all into a new publication on 
my theory, as given by the present article. 
Meaning of the medullary tube. — The starting point, then, 
is the peculiar way in which the central nervous system is 
founded in Chordates. It originates as a tube opening 
anteriorly to the exterior and posteriorly into the archenteron 
and showing at its posterior end certain relations to the 
blastopore and the anus. Several suppositions have been 
made to account for these peculiarities. Thus SEDGWICK 
(1884) proposed the following hypothesis “on the original 
function of the canal of the central nervous system in 
Vertebrates.” First there has been a longitudinal groove, 
which closed to form a canal), open at the anterior and the 
posterior end and having a partly respiratory and partly 
protective function. The water entered into the canal “by 
the anterior pore, was driven through it by cilia, and at 
the hind end passed through the neurenteric canal into the 
alimentary canal and so out by the anus.” 
A similar suggestion was made in the same year by VAN 
WYHE (1884 p. 683). Originally the water from the tube 
and the excrements from the archenteron passed out through 
a common opening. Afterwards, when this opening was 
