15. E M B R Y O G E N E S I S in
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Contrary to what might have been expected, these properties show
that the marginal zone is the only one capable of differentiating
ectoderm and nervous system, as well as the mesodermal structures to
which it normally gives rise. This is represented by Fig. 3, in which
areas of the young gastrula capable of self-differentiation are mapped.
3. The Internal Marginal %pne and the Endoderm
O f these rudiments one is situated on the inside surface of the marginal
zone, against the yolk cells and on the floor of the blastocoel, the other
(the endoderm) is represented by the yolk cells themselves.
These rudiments were located by means of coloured markers; when
isolated, they follow the very same course they would normally pursue
in the embryo. Hence, their differentiation is consistent with their
prospective potency (Figs. 3 and 4). They produce intestine, liver, lungs,
excretory apparatus, gonads, heart, blood, or limbs. Their determinaForegut
# * Blood
o Oesophagus, stomach
0 Trunk gut
Heart
O Liver
Kidney
FIG. 4. Potentialities of the anlage of the internal marginal zone and of the floor of the
endoderm. (After Holtfreter, 1938.)