1022-7954/03/3902- $25.00 © 2003
Russian Journal of Genetics, Vol. 39, No. 2, 2003, pp. 105–111. Translated from Genetika, Vol. 39, No. 2, 2003, pp. 157–163.
Original Russian Text Copyright © 2003 by Korochkin, Ryskov.
Biologists have been attempting to understand how
the hereditary information is expressed during ontog-
eny to form the variety of cells and tissues characteris-
tic of a given organism for a long time, at least begin-
ning from the birth of genetics. In the late 19th century,
the outstanding German biologist August Weismann
put forward an elegant hypothesis intended to answer
this question (, see also: Akif’ev, A.P.,
Geny i sud’by
(Genes and Fates), Moscow: Tsentrpoligraf, 2001).
Weismann distinguished two types of cell division in
the course of ontogeny, the equal-heredity and unequal-
heredity divisions. In the case of the latter division type,
the genetic material is unevenly distributed between
daughter cells, which determines the difference
between them and underlies the heterogeneity of the
embryo. According to Weismann, there is a speciﬁc
group of organic molecules (biophores) that are con-
tained in the cell nucleus and migrate into the cyto-
plasm through nuclear pores. Thus, the nucleus plays
the active part, whereas the behavior and differentiation
of the cytoplasm and, hence, the structure and functions
of cells depend on speciﬁc groups of biophores that are
called determinants. The germ plasm contains a com-
plete set of determinants; their number is equal to the
number of homogeneous cell types in an adult individ-
ual. Both determinants and biophores grow and repro-
duce (divide), so that one original determinant is sufﬁ-
cient for any number of similar cells of a given tissue.
All determinants controlling the development of vari-
ous individual traits are grouped into so-called ids
located in the nucleus. During mitosis, ids compose
idants, which correspond to chromosomes. Therefore,
according to Weismann, the differences between cells
that appear during ontogeny are determined by the sort-
ing of heredity units (determinants). These units are
unevenly distributed among different cells and deter-
mine their specialization (Fig. 1). Only germ cells carry
a complete set of determinants and can therefore evolve
into a whole organism. That was the beginning of germ-
line theory, which postulates that, as early as at the ﬁrst
cell division, the cells of the body are divided into the
germ line, where the complete set of determinants is
retained, and the somatic line, where determinants are
speciﬁcally distributed among different somatic cells
The development of the germinal anlage in the
ascarid studied in detail by Theodor Boveri in the late
19th century, was consistent with Weismann’s theory.
The germ cells of
are distinguishable from
somatic cells even at early developmental stages,
because the nuclei of germinal cells contain complete
copies of the chromosomal material, whereas chromo-
somes contained in the nuclei of somatic cells lack their
ends. Boveri found that the diminution (decrease in the
amount) of chromatin consisted of two processes: chro-
mosome fragmentation and loss of chromosome ends.
This process begins at the second cleavage division and
repeats every time when a germ-line cell produces a
somatic cell (Fig. 3). Thus, the chromosomes of the
germ cells of
are complex structures, and some
of their chromosomes are not involved in the develop-
ment of somatic organs and tissues.
However, this type of separation between the germ
and somatic lines is rare. Although this separation
occurs in early embryogenesis, it is seldom accompa-
nied by chromatin diminution. Usually, the ﬁne struc-
ture of the chromosomes of somatic cells is not changed
substantially; hence, all cells of the body have the same
genotype, and there are no grounds to speak of their
unequal-heredity division during ontogeny. Therefore,
as Timofeeff-Ressovsky noted, the main question of
developmental genetics, whose subject is gene action
during ontogeny or the pathway from gene to character,
how, given identical sets of genes in all cells of the
body, the diversity of cells and morphological and func-
tional specialization of tissues and organs are formed.
Two hypotheses or models were put forward in the
1920s and 1930s to explain this phenomenon .
According to one hypothesis, suggested by Thomas
Morgan, although the sets of genes are the same, differ-
ent genes function in different parts of the developing
embryo at different stages of differentiation. Therefore,
organs and tissues acquire ﬁrst chemical and then mor-
phological distinctions (Fig. 4a).
Was August Weismann Right?
L. I. Korochkin
and A. P. Ryskov
Institute of Gene Biology, Russian Academy of Sciences, Moscow, 119334 Russia; e-mail: email@example.com
Kol’tsov Institute of Developmental Biology, Russian Academy of Sciences, Moscow, 117808 Russia
Received October 1, 2002
—A new hypothesis of the genetic regulation of cell differentiation is put forward. The hypothesis is
based on the assumption that the diminution and hyperreplication patterns of repetitive nucleotide sequences
depend on the differentiation pathways of cells and tissues.