212 THE ANCESTRY OF VERTEBRATES 



The activity of the periporal growing zone, manifesting 

 itself in the backward movement of the blastopore and 

 the neurenteric pore, at first interferes with the gastru- 

 lation, which causes the backward directed eccentrical 

 closure of the blastopore, then manifests itself in the backward 

 movement of the slit-like blastopore as stated by us above 

 (which stage lasts only a short time), and only finally in 

 the urogenesis as longitudinal growth of the medullary 

 tube in the way in which we image the stomodaeum of 

 Annelids to have passed phylogenetically into the medul- 

 lary tube of Chordates. 



Difference between Urodelans, Anurans and Selachians — 

 There is then no question of the backward movement of 

 the blastopore =. neurenteric pore stopping in front of the 

 anus and the difference between Anuranand Urodelan con- 

 sequently does not lie in the fact that in the former the neu- 

 renteric pore stops a little before the anus is reached and 

 in the latter o,nly after this has occurred, but in that in Anurans 

 the tube-formation, i. e. the closure of the medullary folds, 

 occurs a little before the anus is reached; in Urodelans, 

 Dipnoans and Cyclostomes only after the neurenteric pore 

 (blastopore) has fused with the anus. This graduated 

 difference evidently again depends on the circumstance 

 that in Urodelans the action of the periporal growing zone 

 is stronger than in Anurans, the activity of the perianal 

 relatively weaker than in the latter. This manifests itself, 

 as stated above, in the medullary plate in Urodelans being 

 developed very strongly, the ventral side relatively little in 

 comparison with the Anurans. The same holds good for Dipnoi 

 and Cyclostomes. Now, as we have seen, the perianal 

 growing zone acts mainly ventrally and on both sides of 

 the (future) anus for the simple reason that, as long as 

 the medullary plate is open, the future trunk ectoderm also 

 lies only ventrally and on both sides of the egg. But in 

 front of the (future) anus also, there seems to be some 

 feeble activity, directed towards the ventral blastopore lip 

 which accordingly is developed more strongly where the 

 perianal growing zone is most active (Anurans, fig. 1, plate), 

 less so, where the perianal growing zone is less active 

 (Urodelans etc., fig. 6, plate). 



Now t4»€ action of this dorsal part of the perianal growing 

 zone is opposed by the periporal growing zone, which 

 pushes the blastopore backwards. It is no doubt due to 



