42 A. A. W. HUBRECHT. 
shaped. We will again encounter this crescent (or “ Sichel”’) 
shape in Sauropsids. But as the embryonic shield increases 
in length the centre of proliferation is equally stretched, and 
out of a crescent shape evolves a double wing-shape, the axis 
between the two symmetrical wings being in the axis of the 
embryo. Along this axis the ectoderm freely produces cell 
material penetrating downwards to the right and to the left 
between the germinal layers and forming what has often 
been designated as primitive streak-mesoblast continuing 
backwards in the median line as the connective stalk 
(Haftstiel). 
Here we encounter an all-important phenomenon, which 
will be better understood when we have also considered it 
phylogenetically, and which consists in the substitution of 
what was at the outset the blastopore by what has later 
developed into the dorsal mouth-slit. The lengthening of the 
tissue which formed the lateral lips of the early blastopore 
has now set in, and the further proliferation of this tissue, 
concomitant with a process of coalescence of the right and 
left halves with reminiscences of the original lumen which 
was the slit-like cavity of the stomodzeum (in the ccelenterate 
stage), brings our original centres of proliferation further 
apart. At the same time the continuity of the tissues is 
never interrupted. 
The accumulation of cell material which represents the 
lateral lips of the dorsal mouth-slit (Riickenmund) naturally 
causes an increase in length of the mammalian embryonic 
shield, during which the shape of the shield generally changes 
from a roundish to an oval or pear-shaped one (see Figs. 
93—95). This lengthening is simultaneous with an in- 
crease in the extension of the lateral mesoblast wings (see 
Fig. 60). For Tarsius I have fully established this a few 
years ago (02, Figs. 54,57, 61,72). And for other mammals 
it has been demonstrated by Bonnet (’97, Figs. 18, 19), 
Keibel (?95, 795), and others. 
As soon as this accumulation of material that reveals 
itself in the increase in length of the embryonic shield has 
