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JEAN J. PASTEELS
animal cortex (cf. Fig. 4). In some cases they may flow along the furrow
and constitute a narrow strip; in other cases, the contact with the
cortex (vegetal or animal) may be restricted to a tiny area (Figs. 4 and 6).
When the area of 'white yolk' is large, a single point of its margin has
the privilege of being nearest to the grey crescent (Fig. 5) ; but if the
yolk contact with the cortex is restricted to a narrow strip or a tiny area
(Fig. 6), the conditions of the different points on the border in relation to
the grey crescent are approximately the same, and two blastoporal lips
appear simultaneously (Figs. 4 and 6).
FIG. 6. Inverted egg of Rana esculenta seen, left, from the vegetal pole, right, from the
animal pole; (a) at blastula stage; (6) at gastrula stage. Asterisks indicate the central
point of the grey crescent noted before the experiment. The two areas of vegetal yolk
being small, two blastopore lips appear simultaneously at both poles.
The combination of two factors is thus necessary for morphogenesis :
(1) the presence of vegetal yolk; (2) an action from the central point of
the grey crescent ; this action is not necessarily local, but may be felt at
some considerable distance, i.e. it has a field-like disposition.
This field and its influence may be best demonstrated by another
series of experiments (Pasteels, 1946) where vegetal yolk has been
displaced as a whole. Eggs of the axolotl, freed from their membranes,
were placed in a stiff gel which made their normal rotation impossible
when they were turned. With the position of the centre of the clear
crescent previously marked, the eggs were turned around an axis
perpendicular to the plane of bilateral symmetry. Using different angles
of rotation, therefore, one may obtain every possible spatial relation
JEAN J. PASTEELS
animal cortex (cf. Fig. 4). In some cases they may flow along the furrow
and constitute a narrow strip; in other cases, the contact with the
cortex (vegetal or animal) may be restricted to a tiny area (Figs. 4 and 6).
When the area of 'white yolk' is large, a single point of its margin has
the privilege of being nearest to the grey crescent (Fig. 5) ; but if the
yolk contact with the cortex is restricted to a narrow strip or a tiny area
(Fig. 6), the conditions of the different points on the border in relation to
the grey crescent are approximately the same, and two blastoporal lips
appear simultaneously (Figs. 4 and 6).
FIG. 6. Inverted egg of Rana esculenta seen, left, from the vegetal pole, right, from the
animal pole; (a) at blastula stage; (6) at gastrula stage. Asterisks indicate the central
point of the grey crescent noted before the experiment. The two areas of vegetal yolk
being small, two blastopore lips appear simultaneously at both poles.
The combination of two factors is thus necessary for morphogenesis :
(1) the presence of vegetal yolk; (2) an action from the central point of
the grey crescent ; this action is not necessarily local, but may be felt at
some considerable distance, i.e. it has a field-like disposition.
This field and its influence may be best demonstrated by another
series of experiments (Pasteels, 1946) where vegetal yolk has been
displaced as a whole. Eggs of the axolotl, freed from their membranes,
were placed in a stiff gel which made their normal rotation impossible
when they were turned. With the position of the centre of the clear
crescent previously marked, the eggs were turned around an axis
perpendicular to the plane of bilateral symmetry. Using different angles
of rotation, therefore, one may obtain every possible spatial relation