694 PATTERNS AND PROBLEMS OF DEVELOPMENT 



which is superimposed on the primary bilaterahty and more or less obht- 

 erates it; the tertiary bilaterahty of each individual is superimposed on 

 the secondary bilaterahty and tends to obliterate both it and the primary. 

 It seems possible, however, that mirror-imaging in members of a pair may 

 be, at least in part, the expression of action on each other, like inhibition 

 or other modification of adjoining sides in various other forms. 



STRUCTURAL CONCEPTS OF DEVELOPMENTAL PATTERN 



The "intimate structure" of some sort, so often postulated as the basis 

 of organismic developmental pattern, has been regarded by some as in- 

 herent in the protoplasm concerned, by others as originating in relation 

 to some factor external to the protoplasm. Resemblances or analogies 

 between organismic structures, cells, and even whole organisms and crys- 

 tals have been pointed out repeatedly; and hypotheses of an organismic 

 pattern essentially crystalline, or in some way resembling crystalline 

 structure, have appeared again and again. The discovery of fluid crystals 

 and supposed resemblances of their behavior to that of living protoplasms 

 seemed to give support to these views. ■'^ Observations with polarized light 

 have given evidence of optical anisotropy in many plant and animal struc- 

 tures, and X-ray analysis is advancing still further our knowledge of the 

 ultramicroscopic structure of biological materials. ^^ However, many of 

 the structures in which evidence of a definite molecular or micellar pat- 

 tern is found are nonliving products of protoplasmic activities, such as 

 cuticular substances, cellulose, and other nonprotoplasmic membranes — 

 shells, skeletal structures, fibers of cellulose or protein constitution, hair, 

 silk, etc. Evidence of ultramicroscopic orientation also appears in certain 

 protoplasmic structures, muscle, nerve, fibrillar connective tissue, vari- 

 ous other fibrillar structures of cells, and in the highly condensed or 

 structurized substance of sperm heads. It is perhaps a point of consider- 

 able importance that this evidence concerns nonprotoplasmic structural 

 products and protoplasms with a high degree of structurization. In some 

 of these structural patterns — for example, connective tissue and bone — ■ 

 orientation of ultramicroscopic particles apparently represents primarily 

 a result of mechanical environmental factors rather than a crystalline 

 space lattice, and pressure and tension are probably concerned in many 

 other such patterns. Evidence of change in pattern of proteins in relation 



3" See, e.g., O. Lehmann, 1911. Resemblances and differences between fluid crystals and 

 organismic structures are discussed in many other papers. 



39 Seifriz, 1935; W. J. Schmidt, 1937; F. O. Schmitt, 1939 and citations. 



