158 
be necessary to test the accuracy of the conclusions reached 
by the application of the above principles. They are as follows. 
Interference of both with the gastrulation. — The ectoderm 
which afterwards has to invest the whole soma — dorsally 
also —lies in a stage like that of figs. 40 a and b (text) 
principally at the ventral and the lateral sides and oniy 
afterwards, by the closing of the medullary tube, extends 
over the dorsal side as well. The production of this somatic 
ectoderm must now evidently be performed by the perianal 
growing zone, as is the case in Annelids. Thus in the 
neighbourhood of the future anus, a short distance behind 
the ventral blastopore lip, mitoses may be expected to be 
most frequent. When, however, the blastopore has closed 
(figs 40, 41 a), the rearward extension of this ventral 
ectoderm comes to an end. If now the perianal growing zone 
continues to be active, a ring-shaped thickening of the ecto- 
derm round the anal 
pit will result. This 
having been observed, 
it appears to me that 
it is here we possibly 
have to look for the 
explanation of the 
ectodermal thickening 
which in the figs. 1, 
zand 3 (plate) are 
seen developing in an 
Fig. 43. Larva of the sturgeon, after increasing degree just 
dn she geit Handbuch, under the anal pit he 
le OE , … and which, as parame- 
1 limit of the ga tion, mit ë : 
of the amste ens 3 Mimit of the dian sections teach us, 
urogenesis. reach forward also at 
Beneath: Diagram ofthe interference the left and the right 
of the gastrulation (a) withtheaction of it. In the axolotl, 
( hpa ndr and the periporal__\vnere the extension of 
the ventral ectoderm 
is so slight, this ectodermal thickening also, though present, 
is yet of very little importance (6 *). The activity of the 
perianal growing-zone seems to die down soon afterwards 
and the ectodermal thickening in the ensuing stages gradually 
disappears again. Somatogenesis has closed simultaneously 
with gastrulation. Were it to continue also after the end of 
gastrulation the anus would ewentually lie somewhere 
T 
E 5 a 
prostomium. 
