108 C. M. CHILD 



larvae in one direction into apico-basally elongated forms, with 

 slight apical development, with little or no trace of longitudinal 

 axis or bilateral symmetry, or, in the extreme case, into a spheri- 

 cal form, with little or no trace of any axis, and in the other 

 into a transversely flattened form with very great apical, ante- 

 rior and median over-development, and the localization of parts 

 differs correspondingly in the two types. The direction of the 

 skeletal rods may be shifted through ninety degrees, or the skele- 

 ton may be completely inhibited or very greatly over-developed. 

 It would be difficult, I think, to find a more complete demonstra- 

 tion of the effectiveness of the metabolic gradients in develop- 

 ment than these form-changes in the sea urchin. We are able 

 to modify, to control," and to predict the changes in form and 

 relations of parts which occur. 



The fact that differential effects on development result from 

 the action of inhibiting agents, not only upon the various stages 

 of development themselves, but upon the unfertilized egg is also 

 important. It may appear, at first glance, that this fact con- 

 stitutes a demonstration of the actual existence of the definitive 

 axial metabolic gradients in the unfertilized egg, but this is not 

 necessarily the case, as a moment's consideration will show. 

 Assuming that no metabolic gradients are present in the unfer- 

 tilized egg, the effect of the inhibiting agent must be the same 

 on all parts of the protoplasm or of its limiting surfaces. If 

 this effect persist after fertilization, the protoplasm is less cap- 

 able of stimulation and therefore, if the metabolic gradients 

 arise de novo during or after fertilization, it is conceivable that 

 differences in metabolic rate in such gradients may be less than 

 in normal eggs. If, on the other hand, recovery from the in- 

 hibiting effect is occurring at the time the metabolic gradients 

 arise, the differential acceleration of metabolic rate associated 

 with the establishment of a metabolic gradient may itself de- 

 termine a differential recovery, since the rate and degree of 

 recovery varies with metabolic rate. In short, a general inhi- 

 bition preceding the establishment of the metabolic gradients, 

 if its effect persist during the period of establishment, may de- 

 termine that the metabolic differential, the metabolic slope of 



