The "organization centre". 3. Segregation and pattern formation in morphogenetic fields.

Abstract

1) Experimental evidence strongly supports the view that the subdivision of organ anlagen into smaller structural units is an autonomous process. 2) Dalcq &Pasteels' (1937, 1938) hypothesis which says that the boundaries between the different areas into which a morphogenetic field differentiates are determined by “Threshold values” in the “potential” of the field in question, is inconsistent with our present knowledge of biochemical reaction systems. Threshold values may only be used indescribing the spatial differentiation of a morphogenetic field. 3) It is suggested that the latter concept be replaced by the concept of “optimal values” in the intensity of a morphogenetic field-specific for the various chemo-differentiations characterizing the different regions into which a morphogenetic field will split up-and that each optimal value isdeterminative for the appearance of one particular differentiation within the morphogenetic field, each differentiation being formed within a certain range of intensity of the field. 4) On the basis of the hypothesis formulated above a “virginal” morphogenetic field will necessarily differentiate into a number of smaller fields of second order, which in their turn will split up into fields of third order, etc. 5) ” Pattern formation” may be defined as the differentiation of a morphogenetic field into an increasing number of smaller fields of higher order of qualitative different chemical and structural composition. Since differentiation of a morphogenetic field proceeds autonomously, the capacity for pattern formation is an intrinsic property of a morphogenetic field. 6) In the light of these considerations it is suggested that a morphogenetic field action is responsible for the first differentiation of the developing embryo into regions with predominantly ectodermal, mesodermal or endodermal differentiation tendencies. 7) The present knowledge about the characteristic pattern formation taking place in the axial mesoderm as well as in the overlying neural plate seems to be in good agreement with the implications of the present morphogenetic field concept. 8) It is emphasized, however, that if the present morphogenetic field concept cannot be characterized further biochemically and biophysically, it will have to be replaced by another concept which can be defined more sharply and is more easily accessible to experimental analysis.