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the rear. In Vertebrates this backward movement goes so far 
that finally the cardiac pore, as neurenterie pore, comes to lie ab- 
solutely at the rear extremity of the soma, just in front of the 
anus. This backward movement is evidently produced by a growing 
zone which has entered into activity at the inner end of the stomo- 
daeum, round the porus cardiacus and which causes the stomodaeum 
to extend more and more to the rear. This growing zone I should 
like to call the periporal growing zone. The longitudinal growth of the 
soma of Annelids on the contrary is produced by a perianal growing 
zone. Both these growing zones now exert their influence as I hope 
to show, in the earliest development of Vertebrates, and things are 
still further complicated by the fact that the activity of both, onto- 
genetically anticipated, interferes with the gastrulation. Further 
researches (pricking experiments, counting of the mitoses) will have 
to test the correctness of the conclusions reached by the application 
of the above principles. They are as follows. 
The ectoderm, which afterwards has to invest the whole soma, 
— dorsally too — in a stage as in figs. la and 2a (text) lies prin- 
cipally at the ventral and lateral sides, and only afterwards, by the 
closing of the medullary tube, extends over the dorsal side as well. 
The production of this somatic ectoderm now must evidently issue 
from the perianal growing zone: in the neighbourhood of the future 
anus, a short distance behind the ventral blastopore lip mitoses may 
to be expected to be most frequent. When however the blastopore 
is closed (figs. 1a, 2a), the rearward extension of this ventral ecto- 
derm comes to an end. If now the perianal growing zone continues 
to be active, a ring-shaped thickening of the ectoderm round the 
anal pit will result. This being observed, it appears to me that it 
is here we have to look for the explanation of the ectodermal thick- — 
ening, which in the figs. 1,°2 and 3 (plate) we see developing in 
an increasing degree just under the anal pit (*), and which, as 
paramedian sections teach us, reach forward, also at the left and 
the right of it. In the axolotl, where the extension of the ventral 
ectoderm is so slight, this ectodermal thickening too, though present, 
is yet of very little importance (5%). The activity of the perianal 
erowing-zone soon afterwards seems to die down and the ectodermal 
thickening in the ensuing stages gradually disappears again. Soma- 
togenesis has closed simultaneously with gastrulation. If it continued 
also after the end of the gastrulation, the anus would eventually lie 
somewhere between the yolk-cell-mass and the extremity of the tail. 
In fishes this case is pretty generally found. As an example may 
be mentioned the sturgeon (fig. 5, text), but many teleosteans might 
