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long as the possibility of a similar relation in Protostomia exists. 
However the theory of Srpewick finds in the development of 
Protostomia just as little support as I hope to show is the ease in 
Tritostomia (Vertebrates). A process of so fundamental phylogenetical 
significance as assumed by Sepe@wick’s theory might be expected to 
have: left more distinet traces in the ontogenetic development than 
are demonstrated by the most careful research of recent investigators. 
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 demon- 
strated. Even in the primitive Polygordius, where as a matter of 
fact the blastopore is divided into two halves by a median con- 
striction, the posterior opening nevertheless closes and the anus 
arises by perforation behind the two teloblasts, which lay 
at the rear end of the blastopore. To me the most probable con- 
ception of the origin of the anus seems to be this, that in a larva 
of, the protrochula-type (Müruuer’s larva of Polyelad, pilidium of 
Nemerteans) the entodermal pouch, which is already turned in 
a backward direction, has applied itself to the ventral body-wall 
and is broken through by perforation, in the same way as occurs in 
Deuterostomia, and that thus the trochophore-larve. has originated. 
So I think the idea of a primary relation between the anus and 
the blastopore for Proto- as well as for Tritostomia should be aban- 
doned. The anus in Proto- as well as in Tritostomia arises by per- 
foration, independent of the blastopore. 
Of the three above mentioned possibilities regarding the relation 
of the anus and the blastopore the second then seems to me, 
both for Proto- and Tritostomia, the right one. The third possibility 
however we find exemplified in Urodelans and apparently also in 
Dipnoans and Petromyzontes, which in their early development so 
closely agree with the former. Let us now invoke the aid of my 
theory for further interpretation. 
According to this theory (Dersman, 1913) the vertebrate is to be 
derived from the Annelid by the stomodaeum growing out back- 
wards so strongly that it extends, as the medullary tube, over the 
whole length of the soma, and, as we shall see, even further still 
(formation of the tail!). For the entrance of the stomodaeum into 
the entodermal part of the gut 1 propose the name porus cardiacus, 
this being the former blastopore. Already during the development 
of Annelids we see this cardiac pore by the lengthening of the stomo- 
daeum travelling backwards into segments situated ever further to 
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Proceedings Royal Acad. Amsterdam. Vol. XIX 
