The topic of this Mosbach Colloquium was meant as a question to begin with. When I started to study differentiation and morphogenesis in Volvox I hoped for a straightforward answer along prepared groove- only to find out that also here things follow Murphy's Law: they were much more complicated than expected! Succour had to be sought. Thus, the idea arose to put this question before a board of experts. Experience would have warned any ex-service man never to utter an idea or else you would be made responsible, and it came as it had to come: I was made impressario of this gremium; I had to assemble the experts. These Proceedings contain their expertise. I cannot even say that I biased it by my picking. In the beginning I aimed at setting different accents by inclination and force of habit. Then, by trial and error, by advice and declination, the programme shaped itself. It eventually gained momentum of which also the size of this volume is indicative. In this volume are printed all the papers presented - with two regret­ ted exceptions - but not the sometimes lively discussion, which clari­ fied and pruned here and there. It would just have made the size too unwieldy. Differentiation and morphogenesis start with the expression of genes. The development programme reels off the genome and is regulated by the position of the appropriate genes. Their structure is in the focus of gene biochemistry since the decisive tools have become available.