Safety testing of compounds is performed on laboratory animals, amongst others the safety of the compound for the developing embryo. This is important because the developing embryo may be more sensitive to compound exposure than adults. However, the use of animals for this purpose is becoming increasingly controversial. Animal-free tests are being developed to reduce and replace animal testing, for example by culturing cells in Petri dishes. In this thesis two of these in vitro tests have been investigated: the mouse neural embryonic test cell test (mESTn) and the human neural progenitor test (hNPT). Our central question was which biological processes could be mimicked in the two tests. This determines for which developmental processes these models can be predictive, and for which not. Expression of certain genes and proteins showed that the stem cells in the mESTn changed into brain cells and supporting cells. We also tested compounds with a known effect on the developing brain. These compounds showed some of these effects in the mESTn. The hNPT was developed within the course of this thesis. This test mimics slightly later processes in brain development, primarily the beginning of brain cells making connections with each other. This process could be disrupted by compounds that have similar effects in human brain development. These two in vitro tests can, together with other cell cultures and computer models, contribute to animal-free safety testing of compounds. More research is needed to better understand these cell cultures and to combine them with other animal-free testing methods.