GASTRULATION AND EARLIEST DEVELOPMENT 205 



from Protostomia, as long as the possibility of a relation 

 between the anus and the blastopore in the latter group 

 exists. This might be expected from SEDGWlCK's well-known 

 theory (1884) which derives the mouth and the anus of 

 Bilateria from the anterior and the posterior extremity of 

 a slit-like Actinian mouth of which the borders coalesce 

 in the middle. It ought then to be possible to trace the 

 concrescence-seam joining mouth and anus, which accor- 

 ding to this theory should run over the ventral side of Annelids, 

 in Vertebrates also in the groove between anus and blasto- 

 pore, that is in the so-called "Afterrinne," the "primitive 

 streak" of ROBINSON and ASSHETON (see above) — not in the 

 hypothetical concrescence-raphe in front of the blastopore, the 

 "primitive streak' of the theory of concrescence, as Lameere 

 (1891) and HUBRECHT (1905) assume in their application 

 of Sedgwick's theory to Vertebrates. Thus the presence 

 of a primary relation between the anus and the blasto- 

 pore in Vertebrates would in no way compel us to derive 

 them with Grobben (1908) from the Deuterostomia, as long 

 as the possibility of a similar relation in Protostomia exists. 

 However, the theory of SEDGWICK finds in the develop- 

 ment of Protostomia just as little support as I hope to 

 show is the case in Tritostomia (Vertebrates). A process 

 of such fundamental phylogenetic significance as assumed 

 by Sedgwick s theory might be expected to have left more 

 distinct traces in the ontogenetic development than are 

 demonstrated by the most careful research of recent inves- 

 tigators. Again and again we see the anus arise as a new 

 formation, by perforation. In Annelids, where primarily 

 we might expect to find evidence of a common origin of 

 mouth and anus, a direct transformation of the rear end of 

 the blastopore into the anus has never been established. 

 Even in the pr.mitive Polygordius, where as a matter of 

 fact the blastopore is divided into two halves by a median 

 constriction (WOLTERECK, 1904), the posterior opening 

 nevertheless closes and the anus is formed by perforation 

 behind the two teloblasts which lie at the rear end of the 

 blastopore. To me, as slated befoi-e, the most probable 

 conception of the origin of the anus seems to be that in 

 a larva of the protrochula-type (MuLLER's larva of a Poly- 

 clad, pilidium of Nemertines) the entodermal pouch, which 

 is already turned in a backward direction, has come in 

 contact with the ventral body wall and has broken through by 



