The models I have been researching belong to what would have been an 18-part series showing the chick embryo, and also specific parts of the embryo, e.g. the heart and the brain, during the first four days of development. Like the majority of embryological wax models to be found in current collections, they were produced by the Ziegler studio of Germany, founded by Friedrich and later taken over by his son Adolf.
This series was the result of collaboration between Friedrich Ziegler and the anatomist Wilhelm His, who invited Ziegler to Basel to teach him how to use wax for modelling, as it was more pliable than the leather and lead he had been using previously. An important feature of the production of this series is that it was the first to make use of a new cutting instrument known as a microtome, thought to have been invented by His in 1866. At this point it should be noted that the developmental history of early cutting devices is poorly documented, and that this invention is sometimes attributed to others, for example the Czech physiologist Jan Purkyně. However, the important thing in this context is the effect it had on the models produced by this collaboration.
A microtome is an instrument that assists in accurately cutting samples into consistently very thin slices, or sections. The specimen would be attached to a sliding table which then moved it to the correct position before the blade was manually brought down to cut it. Different types of microtomes are still important devices in current microscopy, although the original design has been replaced by automatic varieties.
In addition to studying the intact specimen, His used the microtome to section chick embryos across several planes, creating a complete series of sections on which to base the models. After drawing the whole embryo and then the various sections, the main body was formed free-hand, based on the drawing of the whole, before the sectional drawings were used to measure and judge how much wax too add or remove. From the originals Ziegler would then craft a series of models. The benefit of this method was that it allowed the models to display the interior structure of the embryos in more detail, which we shall see was instrumental in making these models more than mere teaching aids.
It is true that the primary purpose of wax models was for teaching university students, or in laboratories to train researchers, doctors and science teachers. Natural embryological specimens were often in short supply, and would have to be viewed under a microscope or at the very least a magnifying glass, which would have been inconvenient for teaching a classroom full of students. While these difficulties could be remedied by using drawings and charts, these models are superior because they have the benefit of showing structures in 3-D, making them easier for the students to understand and later recognise.
However, contrary to a common assumption (which has caused models to be undervalued by historians of science), teaching was not their only important role. This is especially evident in the case of these models, as they were involved in a struggle between His’s mechanical embryology and Ernst Haeckel’s evolutionary embryology. For the Haeckelians, models were still essentially mere teaching aids, but His argued that since models functioned as a record of the researcher’s understanding of the specimen’s structure, they should be recognized as research publications in their own right, over which the author of the model, like that of a written article, holds the intellectual rights to.
This opinion was probably in part the result of His’ realization of how closely embryological models and embryological theory can be related. The His-Ziegler chick embryo series was accompanied by a book authored by His, which used these models, and the more detailed interior structure they displayed, to support an alternative to Haeckel’s phylogenetic embryology. Phylogenetics is the study of evolutionary relatedness, and Haeckel claimed that all vertebrates were the same in the early stages of development. While not denying the fact of evolution, His argued that this did not explain the developmental series, i.e. the forces that transform one stage into the next. He therefore favoured a mechanical view. What has often gone unnoticed by historians is the instrumental role played by the microtome and by modelling in the development of His’ views. He relied on his highly detailed 3-D display of the internal development of the chick as evidence against Haeckel, and in 1874 went as far as charging Haeckel with fraud, claiming he had edited pictures of embryos in order to make them look more similar than they were.