The Questions of Developmental Biology

mitosis, fertilization, and unicellular regeneration (only from the fragment containing the nucleus;
see Chapter 2) "converge to the conclusion that the chromatin is the most essential element in
development."* He did not shrink from the consequences of this belief. Years before the
rediscovery of Mendel or the gene theory, Wilson 1895, p. 4 noted, "Now, chromatin is known to
be closely similar, if not identical with, a substance known as nuclein . . . which analysis shows to
be a tolerably definite chemical composed of a nucleic acid (a complex organic acid rich in
phosphorus) and albumin. And thus we reach the remarkable conclusion that inheritance may,
perhaps, be effected by the physical transmission of a particular chemical compound from parent
to offspring."
Some of the major support for the chromosomal hypothesis of inheritance was coming
from the embryological studies of Theodor Boveri (Figure 4.2A), a researcher at the Naples
Zoological Station. Boveri fertilized sea urchin eggs with large concentrations of their sperm and
obtained eggs that had been fertilized by two sperm. At first cleavage, these eggs formed four
mitotic poles and divided into four cells instead of two (see Chapter 7). Boveri then separated the
blastomeres and demonstrated that each cell developed abnormally, and in a different way, as a
result of each of the cells having different types of chromosomes. Thus, Boveri claimed that each
chromosome had an individual nature and controlled different vital processes.
Adding to Boveri's evidence, E. B. Wilson (1905) and Nettie Stevens 1905a,Nettie
Stevens 1905b; Figure 4.2B) demonstrated a critical correlation between nuclear chromosomes
and organismal development: XO or XY embryos became male; XX embryos became female.
Here was a nuclear property that correlated with development. Eventually, Morgan began to
obtain mutations that correlated with sex and with the X chromosome, and he began to view the
genes as being physically linked to one another on the chromosomes. The embryologist Morgan
had shown that nuclear chromosomes are responsible for the development of inherited
characters.
The split between embryology and genetics
Morgan's evidence provided a material basis for the concept of the gene. Originally, this
type of genetics was seen as being part of embryology, but by the 1930s, genetics became its own
discipline, developing its own vocabulary, journals, societies, favored research organisms,
professorships, and rules of evidence. Hostility between embryology and genetics also emerged.
Geneticists believed that the embryologists were old-fashioned and that development would be
completely explained as the result of gene expression. Conversely, the embryologists regarded the
geneticists as uninformed about how organisms actually developed and felt that genetics was
irrelevant to embryological questions. Embryologists such as Frank Lillie (1927), Ross Granville
Harrison (1937), Hans Spemann (1938), and Ernest E. Just (1939) (Figure 4.3) claimed that there
could be no genetic theory of development until at least three major challenges had been met by
the geneticists:
1. Geneticists had to explain how chromosomes which were thought to be identical in every
cell of the organism produce different and changing types of cell cytoplasms.
2. Geneticists had to provide evidence that genes control the early stages of embryogenesis.
Almost all the genes known at the time affected the final modeling steps in development (eye
color, bristle shape, wing venation in Drosophila). As Just said (quoted in Harrison 1937),
embryologists were interested in how a fly forms its back, not in the number of bristles on its
back.