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of the stomodaeum (medullary tube) exceeds that of the soma, 
so that the cardiac (neurenteric) pore overtakes the anus and 
passes it. Just as in Annelids the position of the anus in 
Vertebrates is terminal in regard to the soma proper, the tail 
being an outgrowth of the dorsal side of the latter in a back- 
ward direction. According to this conception the ventral 
side of the tail belongs to the dorsal side of the soma. 
In conformity with this the dorsal unpaired skinfold of the 
fish- and amphibia-larvae is continued 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, also takes a 
considerable part in the tail-formation. 
Several authors have rightly emphasized the difference 
between somatogenesis and what we may call with DE 
LANGE (1912) urogenesis, i. e. the formation of the tail. 
rom the foregoing results it appears that somatogenesis, 
just as the somatogenesis in Annelids, is performed by the 
perianal growing zone which gives rise to the future somatic 
(not to the neural, which 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. 
Simultaneously, 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, 
ie. the rudiment of the medullary tube. Both growing 
processes are combined with a third one, going on simul- 
taneously: 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; gastrulation and the action of 
the perianal or somatic growing zone. While in Anurans 
both these processes stop nearly at the same time, in fishes, 
as stated above, we frequently find ‘hat 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. The urogenesis 
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. 
