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E T I E N N E
W O L F F
FIG. 7.—Inducing power of a dorsal blastoporal lip, killed by X-rays, on a piece of ectoderm:
differentiation of a cephalic region (with an adhesive filament), of a trunk, and of a tail with
a half-fin. ad, Adhesive filament; f, fin; H, head; T , tail. (Original photograph by M .
Reyss-Brion.)
disappear first; archencephalic differentiations (anterior brain and
related sense organs) are the last to disappear.
3. Absence of Species-specificity among Inducing Agents
Holtfreter has shown further that organs, or extracts of organs of a
great variety of animals exert an inducing action on indifferent
ectoderm analogous to that exerted by the dorsal lip of the blastopore.
Amongst the many experiments carried out by Holtfreter, hundreds of
tissues or of extracts have shown such effects. W e can cite amongst
the most active: the embryonic extract of chicks, coagulated by heat;
the liver, the kidneys, the heart, and brain of mice; thyroid, kidney,
liver and brain of man, the lower layer of sediment after centrifugation
of crushed calf liver, liver of lizard, frog, newt and stickleback. Many
other examples of tissues and substances, taken from both vertebrates and
invertebrates (dragonfly, spurge hawk-moth, Limnea, Daphnia) could
be cited.