One of the most remarkable features of living organisms – plant or animal – is the way in which a single-celled zygote, the fertilized egg, is transformed by cell division, cell differentiation and cell movement into the adult form, far more complex and often millions of times larger. Development (often called embryology when it describes the progress from egg to a young but free-living stage) is a most challenging field of research and there has been some remarkable progress in recent years. New insights help us to understand how the sequences of development have been modified as a group of animals has evolved, for example as the vertebrates have evolved through fishes, amphibians and reptiles to birds or mammals. The initial stages must stay the same and modifications can be added only at the end. Thus all vertebrate embryos go through a stage which involves their developing gill clefts. Fish – of course – retain these and so do some amphibians but in reptiles, birds and mammals they close up as other structures pertaining to their future life on land are added on and replace them. Hence the old phrase, ‘ontogeny recapitulates phylogeny’ which retains its power if not taken too literally.
Young animals grow up
The zygote contains all the information necessary to build a new organism, provided that it can develop in and interact with a suitable environment. When we study the development of behaviour, we must obviously concern ourselves with some aspects of embryology – for example, the way in which the basic framework of the nervous system is laid down – but we need to go far beyond this.