1 5 . E M B R Y O G E N E S I S in ditto
535
taining presumptive material for caudal somites or lateral plate sometimes differentiates only into epidermis and connective tissue: however,
the formation of epidermis shows its regulatory power. The whole of the
chordo-mesodermal zone constitutes a
4
'gradient field" or a "morphogenetic field" (earlier embryologists called it a "harmonious equipotential system"), characterized by three essential properties: (1) the
regulation of missing parts and the tendency to reconstitute a whole
system; (2) the subordination of parts to a dominant region and the
formation of a hierarchy of differentiations; (3) the decrease in regulatory and differentiating power along one or two axes.
It may be noted that explants of the somite and chordal region have
a tendency to form a more or less complete embryonic axis (Fig. 2)
FIG. 2. Explantation of a fragment of the somite anlage (left). Result of the explantation (right):
notochord, neural tube (with encephalic vesicle), epidermis, muscles, connective tissue.
(After Holtfreter and Hamburger, 1956.)
with a cephalo-caudal organization and bilateral symmetry. In particular the neural tube which appears above the notochord swells into
a cerebral vesicle at the front and is pulled out into spinal cord at the
back.
The external marginal region, therefore, possesses some remarkable
properties: besides its capacity to differentiate organs which are
characteristic of this region and which conform to its normal potential
function, it can widen its potentialities and give rise to other structures:
namely, it can produce the mesodermal organs which it lacks and also
differentiate ectodermal derivatives (nervous system-epidermis). This
region is capable not only of regulation but also of self-differentiation.
It does not simply complete that which it lacks but also organizes
differentiations inside its own territory—and one may note that there
already exists a certain regional differentiation along a cephalocaudal axis.
535
taining presumptive material for caudal somites or lateral plate sometimes differentiates only into epidermis and connective tissue: however,
the formation of epidermis shows its regulatory power. The whole of the
chordo-mesodermal zone constitutes a
4
'gradient field" or a "morphogenetic field" (earlier embryologists called it a "harmonious equipotential system"), characterized by three essential properties: (1) the
regulation of missing parts and the tendency to reconstitute a whole
system; (2) the subordination of parts to a dominant region and the
formation of a hierarchy of differentiations; (3) the decrease in regulatory and differentiating power along one or two axes.
It may be noted that explants of the somite and chordal region have
a tendency to form a more or less complete embryonic axis (Fig. 2)
FIG. 2. Explantation of a fragment of the somite anlage (left). Result of the explantation (right):
notochord, neural tube (with encephalic vesicle), epidermis, muscles, connective tissue.
(After Holtfreter and Hamburger, 1956.)
with a cephalo-caudal organization and bilateral symmetry. In particular the neural tube which appears above the notochord swells into
a cerebral vesicle at the front and is pulled out into spinal cord at the
back.
The external marginal region, therefore, possesses some remarkable
properties: besides its capacity to differentiate organs which are
characteristic of this region and which conform to its normal potential
function, it can widen its potentialities and give rise to other structures:
namely, it can produce the mesodermal organs which it lacks and also
differentiate ectodermal derivatives (nervous system-epidermis). This
region is capable not only of regulation but also of self-differentiation.
It does not simply complete that which it lacks but also organizes
differentiations inside its own territory—and one may note that there
already exists a certain regional differentiation along a cephalocaudal axis.