1269 
also be mentioned here, in whose larvae the place of the anus varies 
much and is of importance in determining the species. 
Let us now turn to the periporal growing zone, which causes the 
growing out of the stomodaeum, resp. the medullary tube, resp. the 
medullary plate, together with the backward movement of the cardiac 
pore (Annelids), resp. the blastopore, resp. the mneurenteric pore 
(Chordates). Organs or processes that are of much importance for 
the structure of the adult animal, in ontogeny often appear preco- 
ciously. In Lamellibranchia e.g. the shell-gland invaginates already 
during gastrulation, though the latter process phylogenetically is no 
doubt much older. Thus also the activity of the periporal growing 
zone, and the backward movement of the cardiac pore associated 
with it begins very precociously, viz. already during gastrulation, 
when the future cardiac pore is still the blastopore. The interference 
of the contraction of the blastoporic rim with the backward move- 
ment of the blastopore causes the caudadly excentric closure of the 
blastopore, which is typical for chordates. The activity of the 
periporal growing zone, as long as the tubeformation has not 
set in, results not in the production of a stomodaeal viz. me- 
dullary tube, as is the case afterwards during the urogenesis, 
but provisorily in the formation of the medullary plate. The 
growing out of the stomodaeum to the medullary tube is thus in 
its first, somatogenetic part to be imagined projected on a 
plane, the dorsal plane of the embryo. When the blastopore has 
narrowed to a slit and the tube-formation sets in in the form of the 
medullary folds, the caudad wandering of this slit-like blastopore, 
as stated above, continues nevertheless, truly only over a little 
distance — indeed in view of the short duration of this stage 
nothing else could be expected — and so probably with undiminished 
speed. Further than the anus however this backward movement can- 
not go, phylogenetically: the stomodaeum of the Annelid, growing 
out backwards, at last reaches the anus. If now the movement stops 
a little in front of the anus, there will be no relation whatever 
between neurenteric pore (blastopore) and anus (fig. 4a, text), as we 
stated in the frog. [f the movement continues yet a little further 
(fig. 4b), a secondary relation between neurenteric pore (blastopore) 
and amus results.') The anus now opens to the exterior through the 
hindmost extremity of the medullary tube, from the medullary canal 
one can pass through the anus to the exterior as well as through 
1) In a longitudinal section as in fig. 4 the constellation at first sight might 
appear in fig. 4b radically different from that in 4a. If however one imagines 
things in space, the agreement between them will be evident. 
81* 
