The answer in Harrison's case would appear to rest largely on the notion of a biological field active in the cell during growth and development. Haraway found the image of the field important enough for the book's title and first glosses the metaphor in her discussion of Harrison:
"Area boundaries overlap, and tissue of an intermediate region is organized into one organ or another as a function of the center whose influence predominates. Such systems came to called fields, but it is not a word used by Harrison in that context until the late 1930s. As Waddington cautioned later, the term field should convey more than a geographical meaning; he suggested a term such as area or district when one does not intend to refer to the complex of processes involved in organ formation ... Harrison's discussion of structures and processes involved in axis determination of the limb and ear is an analysis of the nature of a field and constitutes one of the first and most basic of such studies. Harrison did not use the word field very often and especially not as a deliberate theoretical concept as Weiss would have done; but nonetheless, it was his fundamental work that first gave concrete content to the organicist notion." [80]
As to Harrison's understanding of how fields might operate, the answer would appear to involve an appeal to liquid crystals:
"Some kind of paracrystalline organization, specific for a type of cytoplasm, would underlie form relations and form changes -- morphology and morphogenesis. Progressive orientation of protoplasmic elements (restriction of degrees of freedom) could account for polarity and symmetry, without any need to postulate the cell as a homogeneous system. Crystal organization in organisms was itself an example of an intermediate level of organization, joining processes of organic and inorganic nature. And perhaps most significantly, Harrison's use of crystal analogies allowed him to bypass assumptions of the mosaic-mechanistic theories of development about part-whole relations and to account for the existence of equipotential systems without turning to either entelechies or classical machines." [93]
Crystals and liquid crystals appear and reappear throughout the book. Their mention here in reference to Harrison's understanding of the operation of development fields stands out as the identification of an intermediate level of organization between the atomism of molecular biology and the whole organism of an earlier vitalism or the later organicism.
What of the edges of things?
Harrison apparently wasn't much of an attention-seeker when it came to his own work, but the following would seem to have been downright prophetic of the coming work in epigenetics:
"Birefringent material was seen in Harrison's material, especially at cell membranes. Harrison noted that their work was preliminary and should be followed up systematically. "Especially should the cell boundaries be examined thoroughly, for it is there, perhaps more than anywhere else in the cell, that we may expect to find the seat of directive forces" ... A glance at current journals in cell and developmental biology reveals the appropriateness of the admonition to study cell membrane systems in relation to form problems." [82]
Material regarding Needham and Weiss to follow in later posts.
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