Vertebrate Limb
Earlier we know that how a series of sequential and coordinated interactions between different cell groups and tissues carry about the construction of a complex organ, the vertebrate eye. In this we will study the development of other complex organ, the vertebrate limb. The paired limbs of all tetrapod vertebrates are made on a common basic pattern, develop in similar manner from cells derived from identical sources in the embryo and, hence, they are all homologous structures. An extremely large number of descriptive and experimental studies have been done on development in vertebrates throughout the present century.
At first the work was done mostly on amphibian embryos but later the chick has been chosen for, among other reasons, the chick embryos are large, easily accessible and obtainable throughout the year. As recently much experimental work has been done on the development of wing of chick embryos. The wing bud is comparatively a large structure and can be manipulated in several ways by the investigator. Portions of the bud can be cut out and their development studied through grafting them in the same or different embryos or on chorioallantoic membrane of older chick embryos. Its mesodermal and ectodermal components can be separated from each other by treatment along with some chemicals like trypsin, EDTA etc or microsurgery and then they can be reassembled readily in different combinations. The two components of such type of synthesized buds can be from the same or different limb types, from embryos of similar or different stages, of the same or different species. The development of such types of combinations can be studied by grafting them on embryos or chorioallantoic membrane or culturing them in vitro. Much of our understanding about the mechanisms involved in limb development has come from investigations on such studies on chick embryos.