EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 69 
in immediate contiguity with it, separate into ecto- and 
entoderm by delamination.! 
The result of this delamination is the mammalian gastrula 
—sometimes characterised by the temporary presence of an 
actual blastopore—which very soon undergoes a series of 
developmental changes (different from gastrulation) to which, 
in this incipient stage, the name of notogenesis may be 
applied. The tissues further forwards, that were the first 
to appear, are simultaneously contributing to what may be 
called kephalogenesis. The budding of the trunk from what 
becomes the extreme forward region of the head is designated 
by these two terms. 
Before notogenesis has commenced, a posterior portion of 
the ectodermal embryonic shield (immediately behind the 
spot where the blastopore is actually or only virtually situated) 
is told off as the mother-tissue of what will be the ventrai 
mesoblast. When notogenesis is inaugurated it does so by a 
marked median and ventral proliferation of the ectoderm in 
front of the posterior region just alluded to. This pro- 
hferating downgrowth (the protochordal wedge) becomes 
confluent with the entoderm and fuses with a proliferation in 
it (the protochordal plate). 
Both proliferations are centres of origin of mesoblastic 
and mesenchymatic tissues. ‘The antero-median entodermal 
proliferation, called the protochordal plate, is in continuity 
with a ring-shaped area stretching sideways and closing 
behind, below the ventral mesoblast. This ring is the place 
of origin of blood and blood-vessels. In certain mammals it 
contributes in its hinder portion to a very early vascularisa- 
tion of the trophoblast, thereby calling forth a connective 
stalk (Bauchstiel), in which a remnant of entoderm, drawn 
out in tube form, is the first indication of what in less 
primitive arrangements has come to be the allantois. 
1 The two germ-layers (ectoderm and entoderm) of all the Vertebrates arise 
hy delamination ; only in Amphioxus they owe their differentiation to a pro- 
cess of invagination so common among invertebrates. 
