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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 Anuran and 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 only 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). É 
Now the 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 
