Crosses between diploid raspberry cultivars (Rubus idaeus L.) and their autotetraploid forms have shown that embryo shape depends on three main factors - the variety, the stage of development reached before growth ceases, and the ploidy of the embryo itself. Embryo growth, however, depends on the ability of the endosperm to nourish the embryo and so on the harmonious co-existence of derivatives of the gametes which formed the endosperm. Crosses between plants of unequal ploidy produce endosperms which are unbalanced. In general, the order of fertility of the crosses is 2n selfed, 4n selfed, 4n x 2n, 2n x 4n, and this can be partly explained if the effect of chromosome doubling is to increase the genetic strength of the male gametes more than that of the female. Variations in the embryo growth in the 2n x 4n and 4n x 2n crosses are ascribed to smaller differences in genetic strength between varieties and between the male and female gametes within a variety. Embryos derived from crosses within a single variety grew less well than those derived from crosses between varieties, other factors being equal, and this is considered to be an early expression of inbreeding depression in the embryo.