Peritectic alloys form a variety of solidification morphologies of which peritectic coupled growth (PCG) is the most peculiar. Because the conditions that lead to PCG are still under discussion, in-situ investigations of the evolution from lateral bands to PCG were carried out by using binary organic TRIS-NPG model alloys at concentrations in the peritectic region. The experiments were done vertically with relatively large rectangle glass tubings. For a hyper-peritectic alloy at growth rates below the morphological stability limits of both solid phases, we found that first the properitectic phase grew while lateral bands of the peritectic phase formed between the glass walls and the properitectic phase. Then, the peritectic phase dominated the growth until lateral bands of the now properitectic phase occurred, again close to the glass walls. Now both phases grew side-by-side as two layers at different depths in the sample. However, the properitectic phase spread locally onto the peritectic/liquid interface, so that patches of peritectic phase surrounded by a properitectic matrix developed. This resulted in creating an unstable PCG mode with cycles of increasing and decreasing phase amounts, which from a side-on view resemble tulip- and onion-like structures respectively.