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, Ross Granville Harrison (1937), Hans Spemann (1938), and Ernest E. Just (1939), 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.

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.

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.