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over the tip and the underside of the tail as far as the anus. The 
mesoderm originating at the blastopore-border, and evidently being 
a product of the periporal growing zone, this too takes a conside- 
rable part in the tail-formation. 
De Lance (1912) rightly emphasizes the difference between soma- 
togenesis and urogenesis, though I cannot concur with him in his 
conceptions on gastrulation and mesoderm formation, as expressed 
by the words cephalo- and somatogenesis. From the foregoing results 
it appears that somatogenesis, just as the somatogenesis in Annelids, 
is produced by the: perianal growing zone, which gives rise to the 
future somatic (not the neural, that is that of the medullary plate) 
ectoderm of the trunk, which, as long as the medullary plate is 
open, lies mainly ventrally and at the sides of the egg. Simul- 
taneously, however, with the gastrulation the periporal growing zone 
is at work, which produces the backward movement of the blastopore 
and the backward extension of the originally crescentic rudiment of 
the medullary plate — the rudiment of the medullary tube. And 
both growing processes are combined with a third one, going on’ 
simultaneously: the gastrulation, manifesting itself at the surface in 
the contraction of the blastopore border. 
The urogenesis however sets in after two of these three processes 
have finished, viz. the gastrulation and the activity of the perianal 
or somatic growing zone’), and accordingly is exclusively the result 
of the periporal growing zone, which causes an elongation of the 
medullary tube, disproportional to the length of the soma. The 
difference between somatogenesis and urogenesis herein finds an 
explanation. The activity of the periporal growing zone, manifesting 
itself in the backward movement of the blastopore resp, neurenteric 
pore, at first interferes with the gastrulation, which causes the 
backward directed, excentrical closure of the blastopore, then manifests 
itself in the backward movement of the slit-like blastopore, stated 
by us above, which stage lasts only a short time), and later in the 
urogenesis as longitudinal growth of the medullary tube. 
There is then no question of stopping the backward movement of 
the blastopore viz. neurenteric pore in front of the anus (comp. 
fig. 4), and the difference between Anuran and Urodelan consequently 
does not lie in the fact that in the former the neurenteric pore 
stops a little before the anus is reached, in the latter only after 
1) While in Anurans both processes stop nearly at the same time, in fishes, as 
stated above, we fairly frequently find that somatogenesis continues after gastrulation 
has been completed, so that the anus eventually lies somewhere about halfway 
between the yolk-cell-mass and the tip of the tail. 
