Biology Reference
In-Depth Information
chromosomes. Strict determinism ensues in the relationship which
links the phenotype to the genotype, which was already present in
Weismann's work. “
From the moment when the phenomena which
precede segmentation commence in the egg, the exact kind of organ-
ism which will be developed is already determined — whether it will
be larger or smaller, more like its father or its mother...
” (EH p. 104).
This determinism is absolutely necessary for the reproduction of the
phenotype from the genotype (or the body from the germ-plasm).
It has been regularly restated throughout the history of genetics,
right up to the 1960s, with molecular biology and its 'central
dogma' stating that the organism is entirely coded by information
contained in the DNA (see chapter 3).
A second element in the genetic postulate has more important
consequences: the unvarying transmission of the germ-plasm (or DNA
in current genetics) ensures continuity of the species through juxta-
posing identical ontogenesis, each individual produced being a lateral
excrescence of the germ-plasm (Fig. 32). As Weismann put it:
“…in each ontogeny, a part of the specific germ-plasm contained
in the parent egg-cell is not used up in the construction of the body
of the offspring, but is reserved unchanged for the formation of the
germ-cells of the following generation”
(EH p. 170).
These germ cells “
only contain the undying part of the organism —
the germ-plasm
” (EH p. 209), that is to say, its Form, or soul, accord-
ing to Aristotle's terminology. The reproductive material is protected,
in this conception, from the world and accidents. When Weismann
located it in the nucleus of germinal cells, he talked of it as the essence
of the cell.
Genetics thus re-established an Aristotelian theory. The body
no longer reproduces directly from its parts, but from a germ-plasm
of unique origin, sheltered from any fluctuation in the nucleus of
the germinal cells. It represents the whole organism and guides its
ontogenesis in each generation. It acts as the formal cause of
embryogenesis to give the living organism its specific organisation.
It is the equivalent of the soul (Form), of what today we call the
