STOMODAEUM AND MEDULLARY TUBE 



difficulties connected with it, I was not thinking at all of 

 the problem of the origin of Vertebrates nor did I 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 I tried ta 

 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 theoiy were provided by the elaboration 

 of this idea. 



After the first publication in \9\3(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 w^ere 

 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 b^Dc. 



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 



