ORIGINS OF EMBRYONIC PATTERNS 701 



terms of concentration gradients it becomes necessary to assume that 

 they undergo change in position and concentration relative to each other 

 in orderly and definite ways; one must "suppress" the other, steepness 

 must change, and concentration must decrease or increase. The assump- 

 tions made often seem to be concerned primarily with activity gradients 

 and their changes, though stated in terms of concentration gradients. In 

 any case, metabolism seems to be necessary to bring about the postulated 

 changes in the concentration gradients. If a single metabolic gradient 

 may be associated with two opposed substance gradients, it, rather than 

 the substance gradients, is the effective factor in developmental pattern. 

 Concentration gradients cannot accomplish development without metabo- 

 lism. Also, differences in concentration of substances in different regions 

 may result from differences in metabolic activity. If we substitute rate 

 or intensity for concentration, many experimental modifications of de- 

 velopment present less difficulty to interpretation. In these terms many 

 lateral asymmetries and their reversals appear rather similar to cases of 

 physiological dominance of one side of the body over the other; something 

 of this sort seems to be what Ludwig has in mind when he suggests that 

 injury of the previously predominant agent may result in predominance 

 of the other and so bring about reversal of asymmetry. 



At first glance the asymmetries, particularly those with more or less 

 constant laterality, may seem to require an underlying molecular or other 

 spatial structural pattern, genetically determined. But molecular orienta- 

 tion alone cannot determine asymmetry on an organismic scale. As Har- 

 rison has recognized, there must be another factor bringing about locali- 

 zation of different developmental potencies on a larger than molecular 

 scale. However, many lateral asymmetries become evident at different 

 developmental stages in different parts. Moreover, they behave experi- 

 mentally like gradients, not like specific localizations; they show differ- 

 ential susceptibility to external agents, and some of them can be obliter- 

 ated and reversed in the same ways as other features of axiate pattern. 

 Is it perhaps possible that the genetic factor in these asymmetries con- 

 sists not in determining an asymmetric structure in the egg but in a defi- 

 nite and constant relation of the developing oocyte to some inequality or 

 shift in conditions in the parent body determining ventrodorsality or dor- 

 siventrality? Or does presence of polar and ventrodorsal gradient in some 

 way determine pattern in a third dimension? Determination of a third 

 axis or differential by passage of an electric current in a conductor through 

 a magnetic field oriented at right angles to the conductor is cited by Hux- 



