702 PATTERNS AND PROBLEMS OF DEVELOPMENT 



ley and De Beer as an interesting analogy in a physical system to deter- 

 mination of an asymmetry in relation to other axes already present in 

 organisms. That it is anything more than an analogy they do not main- 

 tain.^' 



It is sufficiently evident from preceding chapters that gradients of ac- 

 tivity of various kinds — metabolism, rate of cell division, growth, differ- 

 entiation, etc. — -are very generally characteristic of developmental pat- 

 terns. Whatever our assumptions concerning "intimate structure" of one 

 kind or another, or concentration gradients, the earliest distinguishable 

 and most general form of axiate pattern appears to be a gradient or gra- 

 dient system on a molar scale, involving the essential activities of the 

 living protoplasm in which it appears. Within the more general systems 

 of earlier stages new, more restricted gradient systems of different kinds 

 arise as activities at different levels of the earlier systems become increas- 

 ingly different. These activity patterns may bring about definite and per- 

 sistent patterns of molecular or other structure or orientation, crystalline 

 or micellar. In short, patterns of many kinds may result from the orderly 

 and integrated activities of living protoplasms. 



It is evident from earlier chapters that a persistent axiate pattern can 

 originate in a local activation or from a quantitative environmental differ- 

 ential and that at different levels of a primarily quantitative pattern dif- 

 ferentiations and new patterns may arise. There is also considerable evi- 

 dence indicating that ventrodorsality and dorsiventrality are primarily 

 gradient patterns, becoming effective at a later developmental stage than 

 polarity, and after the polar pattern has brought about more or less ma- 

 terial alteration along its course. According to this concept, regional dif- 

 ferentiations, concentration gradients of substance, specific chemical re- 

 lations of regions by means of hormones and other substances, and what- 

 ever molecular orientation or crystalline structure may be present are all 

 developmental expressions, not primary features of pattern. 



The most general and primitive type of pattern on an organismic scale 

 is an excitation-transmission gradient in, or on the surface of, a proto- 

 plasm, resulting from transmission with an intensity decrement from a 

 region primarily excited by an external energy. This pattern can arise 



'" As they point out, when a conductor carrying an electric current and a magnetic field of 

 independent origin are oriented at right angles to each other, the conductor is subjected to 

 a force acting in a direction at right angles both to it and to the magnetic field (Huxley and 

 De Beer, 1934, p. 79). With the magnetic field vertical, the north pole upward, and the con- 

 ductor carrying current horizontally away from the observer the conductor, tends to be dis- 

 placed to the left. 



