EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 51 
thus located in the Amphibia (the same holds good for Hypo- 
geophis) at the spot where the so-called dorsal lip of the 
blastopore makes its first appearance. And this proliferating 
spot (as was already noticed in Mammals and as Brachet 
(702, 703], Bellonci [’841, and Lwoff [’94] have observed it 
for Amphibia), travels backwards a certain distance over the 
surface of the egg, spinning out at the same time both noto- 
chord and somites. 
In the frog I have in the reproduction of Brachet’s figures 
indicated the corresponding regions by the letters Pp and 
Pw. Pp points to most decided entoderm which has by 
delamination become separated from the ectoderm situated 
above it. And the proliferation of ectoderm marked Pw 
and fused (more markedly yet than in the somewhat earlier 
phase of Axolotl) at the very outset of its proliferation 
with the protochordal plate entoderm below it—about in the 
same way as we encounter the same phenomenon in Fig. 48 
for Tarsius—has here already progressed a certain distance 
backwards. This distance has lengthened and the derivates 
of what was originally the protochordal wedge have increased 
in Fig. 80 (Rana), and yet the letters Pp leave no doubt but 
that they are directed towards the original entodermal pro- 
liferation. So does Pw show us what has become of the 
ectodermal proliferating centre. We can quite understand 
that also concerning Amphibia dissensions have existed as to 
whether the notochord—first and foremost derivate of Pw— 
was of entodermal, mesodermal or ectodermal origin. And 
the different authors that have successively sided for the one 
or for the other of these solutions have pronounced them- 
selves as best they could upon a material that is so far away 
from the extraordinary clearness with which these very early 
phenomena reveal themselves to usin Mammals. The con- 
tinuity in which from the beginning ectoderm and entoderm 
pass into each other (Fig. 79) all along the ring-shaped zone 
of delamination (marginal zone of Goette), has contributed so 
extraordinarily to propagate and to strengthen the error of 
those who upheld that the phenomena which we have just seen 
