The Questions of Developmental Biology

3. Geneticists had to explain phenomena such as sex determination in certain invertebrates (and
vertebrates such as reptiles), in which the environment determines sexual phenotype.
The debate became quite vehement. In rhetoric reflecting the political anxieties of the late
1930s, Harrison (1937) warned:
Now that the necessity of relating the data of genetics to embryology is generally recognized and
the Wanderlust of geneticists is beginning to urge them in our direction, it may not be
inappropriate to point out a danger of this threatened invasion. The prestige of success enjoyed
by the gene theory might easily become a hindrance to the understanding of development by
directing our attention solely to the genom, whereas cell movements, differentiation, and in fact
all of developmental processes are actually effected by cytoplasm. Already we have theories that
refer the processes of development to gene action and regard the whole performance as no more
than the realization of the potencies of genes. Such theories are altogether too one-sided.
Until geneticists could demonstrate the existence of inherited variants during early
development, and until geneticists had a well-documented theory for how the same chromosomes
could produce different cell types, embryologists generally felt no need to ground their science in
gene action.
Early attempts at developmental genetics
Some scientists, however, felt that neither embryology nor genetics was complete without
the other. There were several attempts to synthesize the two disciplines, but the first successful
reintegration of genetics and embryology came in the late 1930s from two embryologists, Salome
Gluecksohn-Schoenheimer (now S. Gluecksohn Waelsch) and Conrad Hal Waddington (Figure
4.4). Both were trained in European embryology and had learned genetics in the United States
from Morgan's students. Gluecksohn-Schoenheimer and Waddington attempted to find mutations
that affected early development and to discover the processes that these genes
affected.Gluecksohn-Schoenheimer 1938,Gluecksohn-Schoenheimer 1940 showed that mutations
in the Brachyury genes of the mouse caused the aberrant development of the posterior portion of
the embryo, and she traced the effects of these mutant genes to the notochord, which would
normally have helped induce the dorsal axis.
At the same time, Waddington (1939) isolated several genes that caused wing
malformations in fruit flies (Drosophila). He, too, analyzed these mutations in terms of how the
genes might affect the developmental primordia that give rise to these structures. The Drosophila
wing, he correctly claimed, "appears favorable for investigations on the developmental action of
genes." Thus, one of the main objections of embryologists to the genetic model of
development that genes appear to be working only on the final modeling of the embryo and not
on its major outlines was countered.
*Notice that Wilson was writing about form-building units in chromatin in 1896 before the rediscovery of Mendel's
paper or the founding of the gene theory. For further analysis of the interactions between Morgan and Wilson that led
to the gene theory, seeGilbert 1978,Gilbert 1987 andAllen 1986.
Morgan's evidence for nuclear control of development went against his expectations; until 1910, he was the leading
proponent of the cytoplasm. Wilson was one of Morgan's closest friends, and Morgan considered Stevens his best
graduate student at that time. Both were against Morgan on this issue. Even though they disagreed, Morgan
wholeheartedly supported Stevens's request for research funds, saying that her qualifications were the best possible.
Wilson wrote an equally laudatory letter of support, even though she would be a rival researcher (see Brush 1978).