Development

The Haeckel program in evolutionary morphology, with its descriptive outlook and its tendency to speculate about phylogenetic relationships, left many younger scientists dissatisfied. They sought a mechanistic understanding of development, more in tune with the emphasis on experimentation and causal interpretation that characterized sciences like physiology or chemistry. Championed by Wilhelm Roux, this new approach to the study of development dominated late-nineteenth-century biology in Germany and the United States. In detailed and technically demanding experiments, biologists tested the influence of physical and chemical conditions such as gravitation, pressure, temperature, and varying chemical concentrations in the environment on development of select model organisms (mostly amphibians and marine invertebrates) whose free-living embryos were easy to manipulate. This new experimental program in embryology also benefited from the newly founded marine research stations. Many of these experiments were only possible in well-equipped laboratories in close proximity to the diverse biological material of the sea.

The canonical experimental styles in Entwicklungsmechanik were the destruction of certain parts of the embryo and the transplantation of specific tissues between and within embryos. Both kinds of experiments disrupted normal development and allowed researchers to discover the effects of certain parts of the embryo. Puncturing one of the two cells in a two-cell-stage frog embryo, Roux found in 1888 that only half an embryo developed. In his mosaic theory of development he then argued that during differentiation the determining factors, which are all present in the fertilized egg, are gradually distributed among the daughter cells. In a similar vein, August Weismann argued in 1892 for the separation of the germ line, which he saw as retaining the full developmental potential and being passed on through the generations, and the soma, those elements of an organism that undergo differentiation. Weismann, too, thought that an unequal distribution of hereditary material accounts for the differentiation of cells during development.

When Hans Driesch repeated Roux's experiment, shaking sea urchin embryos apart during the two-and four-cell stages, he observed the formation of complete, albeit smaller, pluteus larvae. Driesch began to think that development could not be interpreted in strictly mechanical terms. The embryos' demonstrated ability to regulate their own developmental sequence led him to argue that organisms are harmonious equipotential systems and not just complex physico-chemical machines. Organisms as individuals are instead characterized by an irreducible telos, their entelechy, that shows itself in their regulatory abilities. Driesch subsequently embraced a form of neovitalism.

The vast majority of biologists, however, did not accept Driesch's philosophizing but remained committed to experimental study of development, mapping cell lineages and investigating fates of transplanted tissues. It was in this context that Hans Spemann and Hilde Mangold found in 1924 that a small region near the dorsal lip transplanted into the ventral side (belly) of a newt embryo could organize a second set of body axes, thus resulting in a "Siamese-twin-like" embryo. They called this specific region of the embryo the organizer, as it was capable of organizing the basic form of the full organism. In addition, researchers demonstrated that interactions between certain tissues such as mesoderm and ectoderm led to the differentiation of phenotypic structures such as the lens of a vertebrate eye, in a phenomenon called induction. The search began for the specific chemical properties of what was assumed to be the material organizer.

It was also clear that ultimately these developmentally active substances would have to be the products of heredity, since the inherited nuclear chromosomes and the genes they presumably carried, together with the material inside the egg, are what is passed on to the next generation. Research programs in developmental and physiological genetics investigated these questions and, after long and painstaking research, could identify specific causal chains, from a gene product to a phenotypic effect. Mutants, such as eye-color mutants of moths and flies, were the preferred experimental systems for this line of research. In 1940 a group headed by the biochemist Adolf Butenandt and the biologist Alfred Kühn were the first to identify and chemically characterize the substance that induced the red-eye phenotype in the moth Epestia kühniella.

After World War II, developmental biology gradually transformed itself into developmental genetics, especially after the techniques of molecular biology allowed researchers to study genes in their cellular context. One of the first genetic systems studied molecularly was the so-called lac-operon system, which regulates the expression of a lactose-digesting enzyme inside a bacterial cell. This focus on regulation continued as more and more regulatory networks of genes were found. In the context of molecular biology, development—the growth and differentiation of an organism—had been redefined as a problem of the regulation of gene expression. Aristotle's epigenesis had given way to the mechanistic preformationism of the seventeenth and eighteenth centuries and had come around again to a more sophisticated blend of preformism through heredity and epigenesis through development.