Since the Russian edition of this book was published in 1975 many new
research works have appeared which have made necessary some additions
for the English edition, to reflect progress in molecular developmental
genetics. Recent important findings in this field have brought about
essential corrections to the concept of genetic regulation of the
process of cell differentiation. The discovery of the mosaic structure
of a gene prompted the re- evaluation of our considerations about the
regulation of gene activity in eukaryotes, and the data about
transcriptional events during ontogenesis are of great importance as
well. Formerly it was generally accepted that a derepression of genes
was responsible for cell differentiation in the process of develop-
ment. Recently three important conclusions have been derived (Davidson
and Britten, 1979) which help to pose the problem in a new way: 1) Only
a small part of single copy sequences of DNA is represented in nuclear
RNA of a given type of cell or tissue: 10% to 20% in sea urchin embryos,
11 % in rat liver, 4% to 6% in Drosophila cell culture, etc. Since only
about 10% of single copy sequences represent the structural genes
(Davidson and Britten, 1973), transcription of almost the whole set of
structural genes occurs.