538
E T I E N N E
W O L F F
tion is very precise from the outset. They represent areas of limited
potentialities. They manifest only feeble regulatory capacities. For
example, there are overlapping territories between liver and stomach.
But, these partial overlappings apart, they form a precise mosaic.
T o form a continuous epithelial sheet, the endodermal cells must
be backed, like the epidermis, by connective tissue, or else they dissociate. But, even when isolated, they differentiate into typical intestinal cells. Hence, the connective tissue is not indispensable for their
cytological differentiation—it is only necessary for their cohesion.
T o sum up, there exist in the embryo three regions of clearly different
potentialities (Fig. 3). The first of these has limited potentialities and
constitutes a fully determined mosaic; it possesses only feeble regulatory
power: this is the entoblast and the internal marginal zone. Its rudiments, when isolated, are capable of self-differentiation but they do
not perform any co-ordinating morphogenesis. The second is the marginal zone and it manifests multiple potentialities. It is almost totipotent except with regard to endodermal differentiations. Hence, it is
fully endowed with the faculties of regulation and differentiation.
However, its initial determinations are labile: it does not constitute a
mosaic of determined territories; arrangement and co-ordination are
always apparent in its differentiations. One might say that an area of
this zone has an obligatory, as well as a facultative, determination.
Thus the chordo-somitic anlage must necessarily give rise to chord and
somites, but it may also give rise eventually to mesonephros, connective
tissue, nervous system, and epidermis. The whole can organize itself
into a coherent ensemble. Thus, this zone has complete regulatory
power; it is a self-regulating system, or in other words, a "morphogenetic field". This system, capable of effecting multiple histogenesis,
also effects a co-ordinated morphogenesis. Finally the third region, the
ectodermal anlage, possesses no determination at all, and is incapable
of either precise self-differentiation or regulation. By itself, it can give
rise only to an atypical epidermis.
Thus we must conclude, surprisingly at first, that only the endodermal
region is precisely determined. The external marginal zone, despite its
decisive role in the organization of the embryo, is not, properly speaking,
a determined zone; at least, its various parts do not have an invariable
determination. But, it alone is capable of differentiating chordomesodermal tissues, and it alone possesses an organizing power which
is similar to that of the whole embryo.
Fig. 5 demonstrates the comparison, according to Holtfreter, between
the prospective fates ("Prospektive Bedeutung") of the anlage of the
young gastrula (Fig. 5a), their self-differentiating possibilities ("Selbsdifferenzierungleistungen"), their overall potentialities ("Prospektive
E T I E N N E
W O L F F
tion is very precise from the outset. They represent areas of limited
potentialities. They manifest only feeble regulatory capacities. For
example, there are overlapping territories between liver and stomach.
But, these partial overlappings apart, they form a precise mosaic.
T o form a continuous epithelial sheet, the endodermal cells must
be backed, like the epidermis, by connective tissue, or else they dissociate. But, even when isolated, they differentiate into typical intestinal cells. Hence, the connective tissue is not indispensable for their
cytological differentiation—it is only necessary for their cohesion.
T o sum up, there exist in the embryo three regions of clearly different
potentialities (Fig. 3). The first of these has limited potentialities and
constitutes a fully determined mosaic; it possesses only feeble regulatory
power: this is the entoblast and the internal marginal zone. Its rudiments, when isolated, are capable of self-differentiation but they do
not perform any co-ordinating morphogenesis. The second is the marginal zone and it manifests multiple potentialities. It is almost totipotent except with regard to endodermal differentiations. Hence, it is
fully endowed with the faculties of regulation and differentiation.
However, its initial determinations are labile: it does not constitute a
mosaic of determined territories; arrangement and co-ordination are
always apparent in its differentiations. One might say that an area of
this zone has an obligatory, as well as a facultative, determination.
Thus the chordo-somitic anlage must necessarily give rise to chord and
somites, but it may also give rise eventually to mesonephros, connective
tissue, nervous system, and epidermis. The whole can organize itself
into a coherent ensemble. Thus, this zone has complete regulatory
power; it is a self-regulating system, or in other words, a "morphogenetic field". This system, capable of effecting multiple histogenesis,
also effects a co-ordinated morphogenesis. Finally the third region, the
ectodermal anlage, possesses no determination at all, and is incapable
of either precise self-differentiation or regulation. By itself, it can give
rise only to an atypical epidermis.
Thus we must conclude, surprisingly at first, that only the endodermal
region is precisely determined. The external marginal zone, despite its
decisive role in the organization of the embryo, is not, properly speaking,
a determined zone; at least, its various parts do not have an invariable
determination. But, it alone is capable of differentiating chordomesodermal tissues, and it alone possesses an organizing power which
is similar to that of the whole embryo.
Fig. 5 demonstrates the comparison, according to Holtfreter, between
the prospective fates ("Prospektive Bedeutung") of the anlage of the
young gastrula (Fig. 5a), their self-differentiating possibilities ("Selbsdifferenzierungleistungen"), their overall potentialities ("Prospektive