Although the terms 'fertility' and 'hatchability' refer in chickens to two biologically independent processes, in practice they are frequently found to be related. This type of association suggests that the two variables are related only through the mediation of a special set of conditions. The latter find their expressions in a low level of fertility and hatchability. It is suggested that under these conditions, a large proportion of 'infertiles' among hatching eggs are cases of pre-oviposital embryonic death. Evidence to support this has been provided by cytological study of the germ discs of such eggs. The germ discs were found to contain colonies of cells, indicating arrested but nonetheless genuine embryonic development. It has been shown, furthermore, that eggs of some hens are unfertilizable. On the basis of the present data it was possible to establish the existence of another peak in the embryonic mortality curve, occurring during the first 25± hr. of zygotic development.