EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 43 
reached a certain stage, an active process of transformation 
is inaugurated, which consists in the visible differentiation 
of all-important organs, notochord and somites, out of this 
matrix. ‘The differentiation becomes first visible at the front 
end where our ectodermal proliferation, the protochordal 
wedge, has grown downwards and has coalesced with the 
protochordal plate. From this point backwards the noto- 
chord is now, so to say, spun out, the so-called primitive 
streak tissues—lateral lips of the dorsal mouth—at the same 
time diminishing in extent. Phylogenetically it corresponds 
to the origin and the coalescence of the lateral lips, not of 
the blastopore, but of the dorsally-situated stomodzeum. 
A comparison between Figs. 983—95 and 96 will at a 
glance reveal the effect of the new state of things. The 
protochordal wedge situated well forwards on the embryonic 
shield of Fig. 95 is no longer visible as such. ‘The fine 
pore that was present just behind it (see the longitudinal 
section in Fig. 98) has undergone a displacement back- 
wards, and has in Fig. 96 attained a position not far from 
the hinder end of the embryonic shield. This is due to a 
very marked process of elongation which becomes perfectly 
evident on the comparison of two longitudinal sections 
through these two embryonic shields (Figs. 98 and 99). 
This process has been known to earlier observers, and has 
been described as the shortening of the primitive streak, 
going parallel to the formation of the earliest somites. How 
the cell material that has arisen as the paired wings of the 
ventral mesoblast and that which is spun out by the moving 
backwards of the protochordal wedge (producing the noto- 
chord in the median axis and the mesoblastic somites right 
and left of it) comes to arrange itself reciprocally and what 
changes are brought about in this material during this 
process is a very difficult and intricate question about which 
the various authors differ. I think we may safely say that 
by the rapid extension backwards of the differentiation 
process, as it is exemplified by Figs. 95 and 96, the dorsal 
region of the trunk is laid down in outlines (hence the word 
