Embryos on day 2 of development should have 4 cells more or less similar in size. In addition, it is also important that each cell has only one nucleus and that there are not too many fragments. When this is not the case, the embryo may not be viable and its development may be stopped.
On the other hand, embryos on day 3 must have 8 symmetrical cells with only one nucleus each. In addition, the pellucid layer should be neither too thick nor too thin.