Thus, before getting into his analysis of the role of ontogeny in phylogeny, de Beer establishes the interactional or coactional nature of what he calls internal and external factors in embryonic development. He notes that ever since the Silurian period, about 300 million years ago, vertebrate animals have had two eyes, as is borne out by the fossil record. Thus, there must be a hereditary or genetic capacity to develop two eyes in vertebrates, and this capacity has been transmitted to every generation for a considerable period. De Beer then goes on to cite others who have shown that internal and external factors cooperate in the production of all characters of an organism. He further exphasizes the distinction between the process of transmission of the internal factors from parent to offspring, and the process of production in the offspring of characters similar to those which were possessed by the parent.
Characters are due to responses, and have to be made anew at every generation.
A change in either the internal or the external factors will result in a departure from normal development. And this historic past is not the phylogenetic line of ancestral adults, but the line of the germ-plasm supplying the fertilized eggs for each and every generation. The action of the internal factors is to ensure that if the external factors are normal and do evoke any response in development and produce an animal at all, that animal will develop along the same lines as its parent. The internal factors are only a partial cause of ontogeny. Child has solved this problem by showing experimentally that the first thing which has to be settled in a developing egg is the polarity, i.e. which part of the egg will give rise to the front and which to the hind end of the future animal.