1
In its axis is seen the dorsal furrow or medullary furrow (a).
2
As the reader will remember, the straight medullary furrow first appears in the middle of the sandal-shaped embryonic shield.
3
On its dorsal side we find in the middle line the straight medullary furrow, bordered by the two parallel dorsal or medullary swellings.
4
This is even clearer a little later, when the medullary furrow is closed into the nerve-tube (Figure 1.93 mr).
5
Hence the primitive groove (Figure 1.128 pr), examined from above, appears to be the straight continuation of the fore-lying and younger medullary furrow (me).
6
Medullary furrow still wide open behind (z).
7
Medullary furrow still wide open from x; greatly widened at z. mp medullary plates, sp lateral plates, y limit of gullet-cavity (sh) and fore-gut (vd).