698 PATTERNS AND PROBLEMS OF DEVELOPMENT 



of factors in addition to the molecular orientation to account for localiza- 

 tion on the molar or regional scale characteristic of organismic pattern. 

 In Harrison's hypothesis the essential point for developmental pattern 

 is not the orientation of the protein molecules but the transport and 

 localization of the substances produced toward opposite poles of an axis. 

 An electrophoretic factor originating from a metabolic differential in rela- 

 tion to attachment of the oocyte could bring about the transport whether 

 the protein molecules were oriented or not, or might determine produc- 

 tion of the substances in situ, and molecular orientation would be entirely 

 unnecessary. In this connection it is of particular interest that an attempt 

 to discover evidence of molecular orientation by application of X-ray 

 analysis to living amphibian neural plate, neural tube, ear, ectoderm, 

 myotomes, notochord, and yolk and various tissues of chick embryos is 

 entirely without positive result (Harrison, Astbury, and Rudall, 1940, 

 "An attempt at X-ray analysis of embryonic processes," Jour. Exp. 

 Zool., 85). The authors conclude that the negative results do not sup- 

 port the theory of molecular orientation but do not disprove it. 



Not only the origin of spatial organismic pattern but many other fea- 

 tures of development are difficult to account for in terms of a primary 

 molecular pattern. For example, how is the reconstitution of apical partial 

 hydranths from short hydranth stem pieces under good conditions and of 

 complete individuals under inhibiting conditions to be accounted for? 

 How can molecular orientation determine that apical halves of early 

 sea-urchin cleavage stages produce only ectoderm in sea water but after 

 treatment with LiCl produce entoderm and mesenchyme? Ventrodorsal- 

 ity and even polarity can be obliterated in embryos or isolated pieces of 

 various organisms by exposure to certain concentrations of inhibiting 

 agents (chaps, v, vi). Moreover, ventrodorsality in echinoderm embryos 

 may be obliterated without obliterating polarity. Hydroid polarity can 

 be determined by difference in oxygen content of the water. In such de- 

 terminations orientation of molecules, if it occurs, must result from the 

 metabolic differential; also, it must be entirely independent of cell bound- 

 aries. In other words, any molecular orientation present in these and 

 many other cases must result secondarily from the metabolic differential 

 pattern. 



The similarity between organism and crystal apparently consists chiefly 

 in appearance of a definite spatial pattern in both; but in the crystal the 

 pattern is geometric, while in the organism regional differences in metab- 

 olism and in concentration and kind of substances arise progressively. 



