206 THE ANCESTRY OF VERTEBRATES 



perforation, in the same way as occurs in Deuterostomia, 

 and that thus the trochophore larva has originated. 



1 think the idea of a primary relation between the anus 

 and the blastopore for Proto- as well as for Tritostomia 

 should be abandoned. The anusinPioto- 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 stomoc'aeum 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 (formation of the tail!). I have proposed the name porws 

 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, 

 perianal, 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 bet that the activity of both, ontosie- 

 netically anticipated, interferes with the gastrulation. Further 

 researches (pricking experiments, counting of the mitoses) will 



