 The fertilized egg cell contains a nucleus made up of chromosomes inherited from both parents. Along these chromosomes are lined up units of heredity that serve like Weismann’s determinants, now called genes. These genes correlate with some characters in the resulting organism, and therefore in some sense the resulting form was predetermined in the egg. Yet it was not already formed. And, indeed, the mere correlation of genes and characters tells us virtually nothing about how the differentiation occurs nor about how form becomes formed (the problem of morphogenesis). Therefore, yes, genetics brings a sort of new preformation or more accurately predeterminism. But that in itself brings a description and a correlation but no explanation. Or so Morgan initially felt, as did his embryologist colleagues. Advances in genetics soon changed the theoretical landscape. In 1934, Morgan posited differential gene activity as the key to understanding organismal development. On his view, genes are inherited, their activity controlled by cytoplasmic factors. The latter are originally heterogeneously distributed in the egg. Differential gene activity so-driven leads in turn to differential cytoplasmic factor distribution, and the cycle continues, influenced by signals from a cell’s environment (primarily neighboring cells). Ideas of preformation and epigenesis are combined in pre-determined genes acting in response to diverse environmental factors. Burian (2005) argues that the old theoretical opposition was reinscribed in the disciplinary separation of embryology and genetics, particularly in the US. In that context, studies of development emphasizing the nucleus endorsed preformationism, while those focusing on cytoplasm and cell lineage differentiation tended to epigenesis. This theoretical rift was healed when research focus shifted from “cytoplasm vs. nucleus” to mechanisms of gene expression (Burian 2005). Theoretical concepts grounded in genetics and molecular biology thereafter increasingly dominated thinking about development.