161 
lary folds do not, as in Anurans, unite, but ledve af 
opening. We have seen that there is a passage from the 
medullary tube as well through the anus to the exterior 
as through the neurenteric pore into the archenteron. 
Afterwards this is more or less obscured by the fact that 
the medullary folds are in such close contact caudally that 
there is here no lumen, no medullary canal (fig. 8, 
plate) — just as in the frog (fig. 5) — and that accordingly, 
as in the frog, the slit-like neurenteric pore would become 
wholly virtual if the rear part did not remain open as the anus. 
Thus only the anterior part of the slit becomes virtual and 
hence the statement of several authors concerning Urodelans, 
Dipnoans and Petromyzontes, that the blastopore passes 
into the anus and a neurenteric canal is wanting, is to 
be explained. The apparent contrast between Anurans and 
Urodelans has thus found a solution. It would cause us 
no Surprise if in an Anuran a similar condition were observed 
as appears to prevail in Urodelans, nor would the reverse 
case — the difference between them not being fundamental 
but only graduated. It would not be impossible that in one 
species at one time the first, at another the second case 
might be realized (comp. DE LANGE and ISHIKAWÁ on _ 
Megalobatrachus!) 
Formation of the tail. —l have spoken above of the 
caudad movement of the neurenteric pore — blastopore 
stopping in front of the anus. In reality, however, there 
is no question of stopping. Although the anus, when it 
has been reached by the cardiac == neurenteric pore, seems 
to afford an insurmountable obstacle for the further back- 
ward growth of the stomodaeum == medullary tube, the 
activity of the periporal growing zone has not yet come 
to an end when the perianal growing zone has stopped 
working There being no room, however, within the soma for 
further extension, a protuberance of the body wall in front of 
the anus results. Into this the stomodaeum = medullary tube 
grows out. This protuberance is the tail-knob 
(fig.44, c). Thus we see the tail of Vertebrates originating by the 
fact of the periporal growing zone continuing its activity 
after the perianal has stopped. In this way the position of 
the anus in Vertebrates is not terminal, as in Annelids, but 
at the root of the tail which overgrows it and which owes 
its origin simply to the presence of the anus. Phyloge- 
netically we have to imagine that the longitudinal growth 
LXXXII HK 
