Developmental physiology, animal models, and the August Krogh principle

The field of development physiology is growing rapidly, and central to its success has been the judicious use of animal models. This essay considers t he concept of the "animal model" and how a sharp focus on such models is us ually helpful, but in some cases can blind us to alternative models that co uld accelerate advancement of the field. The rationale is presented for the intense use of popular animal models in studying vertebrate developmental physiology such as the early developmental stages of the zebrafish, mouse, and chicken embryo. The essay reminds readers of the August Krogh principle - "For many problems there is an animal on which it can be most convenient ly studied" - and suggests that there are undiscovered animal models that c ould be used profitably in future studies of developmental physiology. Four specific areas of animal characteristics are explored - "macroembryony", t ransparency, ecological relevance, and life cycles showing how consideratio n of each characteristic in experimental animals can help advance the study of physiological ontogeny. Potential new animal models based on the above characteristics are presented (e.g. emu, direct developing frog, armadillo) , and examples of data collected from them are presented. The essay conclud es by discussing the "universality" of physiological data from developing a nimals, indicating that the earlier in development, the more similar are bo th qualitative and quantitative characteristics from a wide variety of vert ebrate embryos and larvae. Thus, with caution, development physiologists ca n meaningfully discuss the common physiological characteristics of the "ver tebrate embryo".