ABSTRACT
IT HAS frequently been argued that genes control only the later-developed and more superficial characters of animals and that the development of the basic plan of the body is controlled, not by them, but by the cytoplasm of the egg; and this contention has been hotly disputed by geneticists who seem to feel that it disparages the importance of their subject. We realise now that, as in so many such controversies, both sides are in the right. Undoubtedly within anyone lifetime a great deal of the basic pattern of the body is dependent on the configuration of the cytoplasm of the egg; one need only remember such phenomena as the arrangement of ooplasms in mosaic eggs (p. r06), of gradients in the echinoderms (p. 8S), or of the formation centre in insects (p. I26). There are indeed many cases in which genetic differences in the nuclei can be shown to have an influence even in early stages of development (e.g. in the merogons or hybrids in frogs, p. 358), but they are certainly not all-important. Thus for embryology the cytoplasm is as fundamental as the genes.
