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58   The Versatile Embryo
     The Washington Post, Saturday, August 19, 1967

     Genetic fatalism is one of the most prevalent and destructive
fallacies about human biology.  This doctrine holds that the quality
of an individual is rigidly determined at the time of fertilization.
It ignores the intricate and marvelous processes by which the egg
becomes an independent adult organism.

     Because of these processes, experimental embryology stands next
to genetics as a central subject of the modern humanities, the
scientific studies of the roots of human nature.

     For many years, research in experimental embryology concentrated
on the most convenient laboratory material: some marine invertebrates,
chickens and especially frogs.  But it was hard to perceive the human
implications and parallels of laboratory curiosities, like some of the
strongly formed tadpoles that could be hatched out of variously
treated eggs.  The perversity of nature also caused frustration.
Species whose eggs were convenient for experimental embryology were
almost always hard to rear to maturity for use in refined genetic
studies.

     During the last 20 years, however, techniques have been perfected
for embryological experiments on the laboratory mouse, already widely
used for studies in genetics and related aspects of cell biology,
cancer research and immunology and other experimental work.

     Except for the important difference in brain growth, the
principles of development in mouse and man are basically the same.

     A striking answer to genetic fatalism is the fact that there
need not be even a numerical correspondence between fertilized egg
and adult.  In exceptional circumstances, one egg may produce several
offspring, and several eggs may produce but one.  This, if two eggs or
early embryos are held together, they usually fuse to produce a single
organism.  Conversely, if an early embryo is pinched and separated
into two halves, it will generate two intact offspring, not two
half-organisms.  The production of half-organisms, the result of a
kind of developmental fatalism, is found in some marine worms, but
not in vertebrates.

     Studies of identical twins had long indicated that man resembled
the frog in these development patterns.  Now genetic evidence makes it
clear that identical twins come from a single fertilized egg, which
has split during early development.

     New experimental evidence comes form Dr. Beatrice Mintz of the
Institute for Cancer Research, Philadelphia.  She has developed the
technique of fusing early embryos of the mouse, then implanting them
in the uterus of a foster mother.  These complexes often develop
normally, and a viable litter is delivered at term.  When inbred
strains of different coat color are combined, the adult animals
display clear evidence of their composite origin, two embryos having 
been merged into one adult.

     There is no apparent reason, other than compelling research
interest, to conduct such a drastic experiment in man, but neither are
there, any overriding, obvious, technical obstacles.  There is a 
50 per cent risk that the two embryos being fused are of opposite
sex, and if this is so the fusion produces a hermaphrodite with
imperfect, bisexual development.  Dr. Mintz's experiments may
shed light on this distressing mishap in man, perhaps sometimes the
result of a natural accident of embryonic fusion.  Rare mosaics
of other kinds, such as mixtures of blood cell type, have also been
described.  They may have a similar origin, although the exchange
of cells probably occurs most often at a later stage.

     The technique of embryo fusion should be especially valuable for
contrasting the effects of specific genes which act inexorably within
single cells, with those in which interaction between cells and
tissues plays a more important role.  For example, the primary sex
cells seem to illustrate the first principle, whereas secondary sex
characteristics are very much influenced by hormones secreted by other
tissues.

     Early grafting leads to a compatibility of tissues that cannot
be achieved readily in adult life.  The embryo fusions are therefore
important for studying the biology of tissue grafting and rejection,
so important for organ transplantation. 

     However, the main lesson we learn from these experiments is the
subtlety of the development blueprint.  Left alone, the egg would make
one whole organism.  Under the influence of another, it makes a
perfectly complementary part.  How one plus one can make one is among
the most challenging problems of biology today.

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