The chick embryo – past, present and future as a model system in developmental biology

The embryo of the domestic fowl (Gallus gallus) holds the record as the animal with the longest continuous history as an experimental model for studies in developmental biology, spanning more than 2 Millenia. Throughout this time, it attracted great naturalists, artists, philosophers, and pioneers of biology and stimulated them to think about the most fundamental questions on generation and life like no other organism has ever done, except the human. The ancient Egyptians are documented as having opened hens' eggs at different periods during incubation to observe the progress of embryo development, and by around 300 BC Aristotle undertook careful studies of the morphology of the embryo (as much as he could without the aid of magnifying devices); this can be considered as the first 'scientific' study of embryo development and his work referred to by his followers right up to the 19th Century (Needham, 1959). After the mediaeval 'Dark Ages', the resurgence of an interest in Anatomy and embryo development in the return to the study of the embryo within the egg. At this time the debate between Preformation and Epigenesis was starting to gather momentum and the chick embryo was often a reference to which subscribers to either camp resorted to support their favourite theory. As an attempt to contribute to the debate, William Harvey (1578–1657) observed chick embryos at early stages of development and concluded that the heart was the first functioning organ to develop in the embryo. By observing the motion of the blood through the heart and early vessels, he discovered the circulation of the blood and understood the function of arteries and veins. The ensuing 200 years or so were accompanied by an increase in the number and the detail of anatomical studies of the embryo, each time enriched by a new technical advance. The introduction of histological sectioning and of selective staining methods allowed Pander and von Baer to start to understand the significance of germ layers in development. These pioneers also started to ask questions about causality in development—what mechanisms are responsible for such stereotyped development? Further and increasingly careful histological studies followed throughout the 19th Century, with the most important contributions being made by Rauber and Hensen and culminating in a beautiful histological atlas by Mathias Duval (1889). By the end of the 19th Century, experimental embryology (Entwicklungsmechanik) started to replace simple histologi-cal observation as it became clear that principles could …