159 
between the yolk-cell-mass and the extremity of the tail. 
In fishes this case is fairy generally met with. As an 
example may be mentioned the sturgeon (fig. 43). Many 
Teleosteans might also be mentioned here, in whose larvae 
the place of the anus varies considerably, which is of 
importance in determining the species. 
Let us row turn to the periporal growing zone which 
causes the growing out of the stomodaeum, the medullary 
tube or the medullary plate, together with the backward 
movement of the cardiac pore (Annelids), the neurenteric 
pore (Chordates) or the blastopore. Organs or processes 
that are of much importance for the structure of the adult 
animal often appear precociously in ontogeny. In Lamel- 
libranchia, e.g., the shell-gland invaginates during gastrula- 
tion, though the gastrulation-process is no doubt phyloge- 
netically much older Thus also the activity of the periporal 
growing zone and the backward movement of the cardiac 
pore associated with it begin very precociously, viz: during 
gastrulation, when the future cardiac or neurenteric pore 
is still the blastopore. The interference of the contraction 
of the blastoporic rim with the backward movement of the 
blastopote causes the caudad eccentric closure of the blasto- 
pore which is so typical of Chordates. The action of the 
periporal growing zone, as long as the tube-formation has 
not set in, results not in the production of a stomodaeal 
or medullary tube, as is afterwards the case during the 
urogenesis, but provisionally in the formation of the medul- 
lary plate. As l have expressed before (cf. p 191), the growing 
out of the stomodaeum into the medullary tube is thus in its 
first, and somatogenetic, phase to be imagined as projected on 
a plane, the dorsal plane of the embryo. When the blastopore 
has narrowed to a slit and the tube-formation begins, in the 
form of the appearance of the medullary folds, the caudad 
movement of this slit-like blastopore, as stated above, conti- 
nues nevertheless, probably with undiminished speed, though 
only over a short distance — as indeed might be expected 
from the short duration of this phase. Further than the anus, 
however, this backward movement can not go; phylogene- 
tically: the stomodaeum of the Annelid, growing out back- 
wards, finally reaches the anus. If now the movement stops 
slightly in front of the anus, there will be no relation whatever 
between neurenteric pore (blastopore) and anus (fig. 44 a), as 
we stated was the case with the frog. If the movement continues 
