696 PATTERNS AND PROBLEMS OF DEVELOPMENT 



it.''^ However, it is a long jump from the molecular patterns of cellulose, 

 silk, wool, etc., or from those of muscle, nerve, or connective tissue, or 

 even those of proteins, to the polarity and symmetry of whole organisms; 

 and the categorical statement quoted in footnote 43 certainly goes far be- 

 yond the evidence. 



A fundamental asymmetry of protoplasms has been postulated by some 

 because it has been found in many cases that only one of the two optical 

 isomeres of certain constituents of protoplasms, amino acids, various 

 lipoids, and sugars serves for protoplasmic synthesis. If such an asym- 

 metry is present, it evidently does not necessarily determine an asym- 

 metric organismic pattern, for many organisms show no evidence of asym- 

 metry. Moreover, in many of those that are asymmetric the asymmetry 

 becomes evident only relatively late in development, only in certain organ 

 systems or organs, and always in definite relation to more general features 

 of pattern that are not asymmetric. If molecular factors have anything 

 to do with organismic asymmetry, they seem to be effective only as locali- 

 zation of primordia and structurization progress. 



If this kind of asymmetry is a general property of protoplasms and is 

 concerned in determining organismic asymmetry, what happens when the 

 organismic asymmetry is experimentally reversed — for example, the coe- 

 lomic asymmetry of the sea-urchin larva or vertebrate asymmetry? Has 

 the one isomere been transformed into the other? Does the individual 

 with reversed asymmetry use the other isomere in synthesis? Does ex- 

 perimental obliteration of asymmetry by differential inhibition or estab- 

 lishment of a new asymmetry by an environmental differential result 

 from a corresponding change in isomere pattern? These questions remain 

 unanswered, and there is still the more general question: How can a mo- 

 lecular asymmetry determine differences in metabolism, in rate and 

 amount of growth, and in differentiation on the two sides of an organism? 

 A general asymmetry of protoplasms has not yet been optically or other- 

 wise demonstrated; but, as pointed out in the preceding chapter, molecular 

 factors may be concerned in many of the asymmetries of highly differen- 

 tiated and structurized protoplasms. 



A molecular hypothesis which attempts to account for specific regional 

 localizations has been advanced by Harrison.''-' Calling attention to the 



« Seifriz, 1935; 1936, chap, xv; 1938. In the latest of these papers he says: "Polarity of cells 

 and symmetry of organisms exist in virtue of molecular patterns indicative of the crystalline 

 state." 



■»^ 19366, 1937, and suggestions concerning a molecular basis of pattern in various earlier 

 papers. 



