538 MOLECULAR MECHANISMS OF DIFFERENTIATION 5 



about the possible relationship of such a cortical organization to the RNA coat of 

 gram positive bacteria (Henry and Stacey, 1943) and of animal and plant cells 

 (Lansing and Rosenthal, 1952). The significance of the cortical structure in 

 embryonic development in general has been emphasized more recently by Leh- 

 mann (1945) and Runnstrom (1952). Weiss (1947) has pointed to the possible 

 role of surface interactions in the induction process and several investigators 

 (Holtfreter, 1948a; Spiegel, 1955; Gregg, 1956; Moscona, 1956) have attributed 

 primary importance to specific forms of surface interactions in the aggregation of 

 cells in the course of embryogenesis. 



The preceding discussion suggests a more generalized view of the mechanism 

 of differentiation. From the examples drawn from different fields of study it has 

 become apparent that all approaches are confronted with the same crucial question 

 of the molecular mechanism by which a chemical, a hormone, a microbial trans- 

 forming principle, an enzyme substrate analog, a hapten, some tissue proteins or 

 chordamesoderm, can leave an imprint on the protein forming system of the cell. 

 This should not be interpreted to mean that one could or should expect that 

 further progress in this field will lead to a single comprehensive theory of difTerentia- 

 tion. The numerous consecutive reactions in the transformation of a rapidly proli- 

 ferating tissue to one which exhibits all the characteristics of structurally and 

 functionally differentiated organs will remain complex and highly specific for 

 each tissue and each species investigated. The individual steps in this process and 

 the interaction of the parts of the developing embryo are too specific to be resolved 

 by a single expression describing the overall process (which undoubtedly could 

 be done) without consideration of the components of the system {e.g. "laws of 

 growth"). In the disciplines of the physicist where the components of the total 

 system are of sufficient qualitative similarity, comprehensive theory construction 

 is more satisfactory both as descriptive and as explanatory schemes of thought. 

 In biology and in particular in embryonic development the interaction of qualita- 

 tively different components is of primary importance in an explanation of the 

 observed changes of the total system. The first step toward an explanation seems to 

 consist in an attempt to demonstrate the mechanism of these individual interactions 

 (Herrmann, 1953c). In this sense embryonic development appears as a picture 

 embroidered in a broad canvas with highly irregular shapes and with the entire 

 spectrum of colors, and not as the regular geometric patterns of a physical theory. 

 Embryology will be written as an epic rather than as a sonnet. In the full account 

 of embryogenesis the general theories of physics will be merely tools to exhibit and 

 to appreciate the diversity of the individual phases of the developing systems'. 



^ Work from this laboratory reported in this Chapter was made possible by grants from the 

 U.S. Public Health Service, Am. Cancer Soc, Assoc. Aid Crippled Children, and Muscular 

 Dystrophy Assoc. 



