The influence of the area opaca on the development of the young chick embryo.

The area opaca of the chick blastoderm is generally regarded as being merely the primordium of the yolk sac. Thus it might be expected that during the early stages of development its role would be essentially to grow and to differentiate, rather than to exert any influence on the development of the area pellucida. Such a view would be supported by the fact that pieces of the area pellucida can differentiate in the absence of the area opaca if they are isolated on the chorioallantoic membrane (Rawles, 1936) or in vitro (de Haan, 1964). There are, however, reasons for enquiring whether the area opaca does exert some influence on the area pellucida. The first is that New (1959) has demonstrated that the blastoderm is normally under tension, and that this tension is produced by the peripheral cells of the area opaca which adhere to the inner surface of the vitelline membrane.