Past exam of the history of science supplementary subject of the University of Oxford / 2021 / 5 / Solution

Images and models were not merely illustrations added after embryological knowledge had been obtained. They organized observations, allowed stages and species to be compared, and made hidden structures available for argument. Their usefulness also created a risk: a clear-looking picture could give an interpretation an apparent certainty that the observational preparation did not warrant.

An embryo changes through time, while a drawing or preserved specimen ordinarily shows one stage. Arranging images in a developmental series makes a temporal process visible by combining observations of different specimens. Such a series can reveal orderly changes, but it already assumes that the specimens are comparable and correctly staged. Differences of orientation, growth rate or species cannot safely be treated as consecutive moments in one individual history. Embryology therefore depended on standards for identifying structures and developmental stages, not only on better artistic technique.

Optical microscopy and section images added a second inferential problem. A two-dimensional section is not the entire three-dimensional embryo. A cavity, a fold and two separate surfaces can produce similar profiles in an isolated section. Three-dimensional reconstruction from serial sections combines aligned sections to constrain a spatial interpretation. In a simplified quantitative version, sections of area $A(z)$ give volume $V=\int A(z)dz$, approximated by ${\sum }_i{A}_i\Delta z$. That mathematical reconstruction does not make section thickness, deformation or alignment errors vanish.

Images also mediated theoretical disputes. Karl Ernst von Baer's comparative embryology connected developmental organization with relations among groups rather than treating every embryo as a miniature adult. Cell theory and studies of fertilization changed the structures that investigators looked for and drew. Oscar Hertwig's account of the cell makes nuclear organization and its relation to reproduction central objects of observation. These were arguments about what visible structures meant, as well as improvements in magnification.^[ref][ref]

Experimental outcomes supplied a check on a purely pictorial account. The contrast between mosaic development and regulative development concerns what cells can produce after intervention, not just the positions assigned to them in a normal embryo's diagram. Hans Driesch's separated-blastomere results challenged a simple allocation of fixed developmental parts to early cells. Comparing them with Wilhelm Roux's experiments also requires noticing that leaving a damaged cell in place is not the same condition as removing it. A schematic diagram which treats those interventions as equivalent can conceal the decisive experimental difference.^[ref]

Physical models and reconstructed images could make complex relations easier to teach and inspect, yet their apparent completeness had to remain answerable to specimens and interventions. The historical role of images was consequently productive and selective: they made certain questions possible and certain explanations persuasive. Good embryological use of images required a chain from preparation through representation to inference, with independent observations and experiments testing that chain.