410
CH.
DEVILLERS
a. Distribution of the Epibolic Power in the Embryonic Disc
The study of the germ from the pregastrular stages brings to light the
evolution of the spreading capacity of the enveloping layer: if half of the
germ of a morula or a blástula is removed (Devillers, 1948a) (Fig. 15) the
portion left on the yolk curves into the shape of a crescent and its points
draw together and meet; the germ becomes circular once more. Since
the two points progress at the same speed, the spreading capacity of the
enveloping layer is uniformly distributed. Now if a young gastrula is
cut along its plane of symmetry, the ventral point moves, while the
FIG. 15. Healing of a germ (attached to the yolk) cut along its symmetry plane (from
Devillers, 1948a).
A: morula; Β: gastrula (dorsal zone below).
dorsal point stays nearly motionless. The spreading capacity is mostly
localized in the ventral ectoblastic zone. If the spreading caused experimentally has the same mechanism as the natural epibolic spreading, we
can witness, with the progressive determination of the germ (Luther,
1936), the localization of mechanical properties.
In the gastrula (see plan of 'anläge', Pasteeis, 1936) the epiblast covers
a larger area in the ventral half than in the dorsal half; as the first half
spreads faster than the second half during epiboly, the blastopore does
not close at the lower pole, but clearly on the dorsal side, as noted by
Kopsch (1904). Wülker (1953) has recognized in Rivulus that the epibolic movement starts from the ventral zone and then spreads little by
CH.
DEVILLERS
a. Distribution of the Epibolic Power in the Embryonic Disc
The study of the germ from the pregastrular stages brings to light the
evolution of the spreading capacity of the enveloping layer: if half of the
germ of a morula or a blástula is removed (Devillers, 1948a) (Fig. 15) the
portion left on the yolk curves into the shape of a crescent and its points
draw together and meet; the germ becomes circular once more. Since
the two points progress at the same speed, the spreading capacity of the
enveloping layer is uniformly distributed. Now if a young gastrula is
cut along its plane of symmetry, the ventral point moves, while the
FIG. 15. Healing of a germ (attached to the yolk) cut along its symmetry plane (from
Devillers, 1948a).
A: morula; Β: gastrula (dorsal zone below).
dorsal point stays nearly motionless. The spreading capacity is mostly
localized in the ventral ectoblastic zone. If the spreading caused experimentally has the same mechanism as the natural epibolic spreading, we
can witness, with the progressive determination of the germ (Luther,
1936), the localization of mechanical properties.
In the gastrula (see plan of 'anläge', Pasteeis, 1936) the epiblast covers
a larger area in the ventral half than in the dorsal half; as the first half
spreads faster than the second half during epiboly, the blastopore does
not close at the lower pole, but clearly on the dorsal side, as noted by
Kopsch (1904). Wülker (1953) has recognized in Rivulus that the epibolic movement starts from the ventral zone and then spreads little by