EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 45 
ectoderm and entoderm in the shape of what appears naturally 
as a flattened layer of so-called mesoderm, but what is in 
reality the strictly grouped material for different organs and 
tissues. ‘These have not sprung from the lips of any blasto- 
pore (Urmund), but have gradually come into existence in 
the same ontogenetical order as we must expect them to have 
arisen phylogenetically, “Lhe blastopore has lengthened out 
into the dorsal mouth. This lengthening has been accom- 
panied by a dorso-ventral proliferation of ectoderm (proto- 
chordal wedge) out of which the stomodeeum (notochord) 
arises, and during this time the dorsal mouth-slit has only 
been represented by vestiges. I have already discussed these 
processes elsewhere (05). The dorsal, elongated mouth 
(Riickenmund, 705, p. 363) may thus point to a vermactinian- 
like ancestor (Fig. 160) in which the appearance of notochord 
and coelomic pouches was already foreshadowed by the sto- 
modeeum and the enteric diverticula to which the stomodzeum 
gives access. 
It is far outside the scope of this paper to establish in detail 
the cell-lineages as they may ulteriorly be found to exist, and 
which will some day allow us to ascribe to each of the three 
centres of proliferation here alluded to, its part in the forma- 
tion of the Anlage of different organs and tissues between 
ecto- and entoderm. It should, however, be noticed that 
already in my publication of six years ago (’02, Pls. 8 and 9, 
figs. 59g and 7o/) I have distinctly figured the fact that in the 
posterior region of the embryonic shield a very considerable 
part is played by the entoderm in the development of the 
Jower half of wings of mesoblast of which the upper half 
springs directly from the ectoderm (Figs. 101—103). 
These and many other phenomena will have to be minutely 
studied and established before we can commence our com- 
parative analysis of these processes in Vertebrates. 
But it should be borne in mind that the processes just 
alluded to have already been mentioned on p. 34, when the 
vascularisation of the “ Haftstiel’? was discussed. And that 
in the diagram, Fig. 46, the posterior source of proliferating 
