Imagine a brain in a petri dish in a biomedical laboratory. Not a whole brain, but a part grown with the help of cells. Through this collection of cells, the researchers perform various experiments. It may involve testing different disease models or experimenting with specific drugs to see how the brain cells react. The researchers can also remove, add or turn off specific genes to see how this brain reacts. Through these tests, medical knowledge is produced about the brain, but also about different brain diseases and treatments. Some of this neuroscientific knowledge can give medical understanding for developmental disability. It is knowledge that can be used to create diagnoses and classification systems which strengthen what we as critical disability researchers would call the medical model. These diagnoses and classification systems are not defined entities, but heterogeneous parts in the medical model. They are, one could say, in various ways linked back to the brain cells in the petri dish. The different parts are an assemblage where the medical knowledge about a specific developmental disability are linked to the materiality in the laboratory, as well to the doctor at the hospital or the person seeing the doctor. In this way, we as disability researchers might be able to transform our cultural framework, we use to criticize the medical model and turn this critique to the laboratory. Is the medical researcher´s work with the brain cells in a petri dish a practice that (re)produces not only the assemblage of what developmental disability is, but also the stigmatized and negative identity that many times exists in society around this disability? In this paper I want to elaborate critical disability theory to see if it can be used to understand the knowledge production practices in and outside the biomedical laboratory.