700 PATTERNS AND PROBLEMS OF DEVELOPMENT 



change in conditions, the stream pattern, hke organismic pattern, is 

 highly persistent but does undergo a gradual development which is a 

 continuous equilibration. But under "abnormal" conditions the pattern 

 may change greatly — for example, in a flood or when dammed. That the 

 living organism is more nearly analogous to a system of this kind than to 

 one resulting from a particular molecular patterns seems probable. 



THE VARIOUS GRADIENT CONCEPTS 



The frequency with which the suggestion that physiological polarity 

 is a gradation or gradient pattern of some sort or a stratification of sub- 

 stances has appeared in biological literature is both interesting and signif- 

 icant. A polar pattern of this sort was suggested for the sea-urchin egg 

 and later for the egg of Ascaris by Boveri (19016; 1910a, b). In many 

 papers on regeneration Morgan postulated gradations of formative sub- 

 stances and also suggested gradation of tension. More recently Runn- 

 strom and his co-workers have interpreted their experimental results on 

 sea-urchin development in terms of two opposed substance gradients in the 

 polar axis, another, ventrodorsal gradient, and a left-right gradient pat- 

 tern (pp. 140, 241). A somewhat similar hypothesis has been advanced by 

 von Ubisch (1936a). Even in Harrison's hypothesis discussed in the pre- 

 ceding section molecular orientation serves merely to produce concentra- 

 tion gradients of substance. The hypothesis advanced by Ludwig (1932, 

 pp. 422-27) postulates different R- and L-agents in opposed concentra- 

 tion gradients as a basis for bilaterality and asymmetry, but how these 

 gradients originate and how they are maintained is not considered. A 

 maximum concentration of one of these agents higher than that of the other 

 determines asymmetry; and if the one of higher concentration is injured, 

 the one that was previously inferior becomes predominant, and reversal 

 of asymmetry results. '•^ The inheritance of asymmetry Ludwig regards 

 as analogous to sex inheritance.'*^ It may be questioned whether the data 

 on symmetry require different R- and L-agents and opposed gradients. 

 Bilaterality apparently results from a ventrodorsal or dorsiventral differ- 

 ential, and many right-left differences may result from a single quantita- 

 tive differential between the two sides of the body. If this is the case, 

 resemblance to sex inheritance is less evident. 



In all attempts to account for experimental alterations of pattern in 



45"Kommt durch Schadigung des pravalierenden das bisher inferiore Agens zum tjber- 

 wiegen, so tritt eine Umkehr der Asymmetrie ein" (Ludwig, 1932, p. 424.) 

 46 Ludwig, 1932, pp. 427-30; 1935- 



