ABSTRACT
There are two views of the relative roles which the nucleus and the cytoplasm play in forming the embryo. One view is the working hypothesis of the geneticist according to which the cytoplasm of the fertilised egg is a passive and plastic substrate on which the chromosomes of the nucleus work to mould the form of the new being. In its extreme form this hypothesis ignores the facts of segregation and localisation of formative substances in the egg. The other view holds that the cyto-plasm is characteristic of the species and therefore determines the general form and pattern of the embryo, that in the early stages of development the cytoplasm is supreme in this respect, and only later do the chromosomes induce superficial modifications in the developing individual.
