15, 
perforation, in the same way as occurs in Deuterostomia, 
and that thus the trochophore-larva has originated. 
[ think the idea of a primary relation between the anus 
and the blastopore for Proto- as well as for Tritostomia 
should be abandoned. The anus in Proto- as well as in Trito- 
stomia arises by perforation, independent of the blastopore. 
Secondary relation between anus and blastopore.— Of 
the three above mentioned possibilities regarding the rela- 
tion of the =nus to the blastopore the second then seems 
to me, both for Proto- and Tritostomia, the correct 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. 
Perianal and periporal growing zones. — According to 
this theory the Vertebrate is to be derived from the Annelid 
by the stomodaeum growing out backwards so strongly 
that it extends, as the medullary tube, over the whole 
length of the soma, and, as we shall see, even further 
still (forma'ion of the tail!). Il have proposed the name porus 
cardiacus for the entrance of the stomodaeum into the 
entodermal part of the gut. This is the former blastopore. 
Already during the development of Annelids we see this 
cardiac pore by the lengthening of the stomodaeum travelling 
backwards into segments situated ever further to the rear. 
In Vertebrates the backward movement goes so far that 
finally the cardiac pore, as the neurenteric pore, comes at 
a certain moment to lie at the utmost extremity of the 
soma, just in front of the anus. This backward movement 
is evidently produced by a growing-zone which has entered 
into activity at the inner end of the stomodaeum, round the 
porus cardiacus, and which causes the stomodaeum to extend 
progressively to the rear. This growing zone 1 should like 
to call the periporal growing zone. 
The longitudinal growth of the soma of Annelids, the 
somatogenesis, is, on the contrary, produced by a terminal, 
Pperianal, growing zone. Both these growing zones now 
exert their influence, as 1 shall try to render probable, in 
the earliest development of Vertebrates. A further complic- 
ation is introduced by the fact that the activity of both, ontoge- 
netically anticipated, interferes with the gastrulation. Further 
researches (pricking experiments, counting of the mitoses) will 
