110 C. M. CHILD 



As regards the longitudinal axis and bilaterality in Arbacia, the 

 fact that they do not become visibly effective factors in develop- 

 ment until later stages, and also the fact that they can be ex- 

 perimentally almost or quite obliterated while the apico-basal 

 gradient still persists, suggest that they are less marked as meta- 

 bolic gradients and perhaps determined later than the apico- 

 basal axis. Conceivably they may arise in connection with 

 fertilization through the differential effect on the egg proto- 

 plasm of the positions or paths of the pronuclei and the direction 

 of the spindle axis. But at present no final conclusion is pos- 

 sible, and, as pointed out above, the differential effects on later 

 development of the action of inhibiting agents on unfertilized 

 eggs do not constitute a demonstration of the existence of the 

 definitive axial gradients in the unfertilized eggs. 



Attention has already been called to the fact that some of the 

 differentially inhibited forms resemble, in certain respects, the 

 more primitive types of echinoderm larvae as regards the com- 

 plete closure of the blastopore, the apico-basally elongated, 

 more or less cylindrical, conical or ovoid body without strongly 

 marked antero-posterior axis or bilaterality, and the develop- 

 ment of the basal portion of the ciliated band as a more or less 

 complete basal ring (p. 79). It may also be noted that the 

 wide-angled forms resulting from differential acclimation show 

 some resemblances in form to certain ophiurid plutei. These 

 resemblances suggest that the condition of the axial metabolic 

 gradients in the experimentally produced forms of Arbacia 

 and the normal forms which they resemble, is somewhat similar. 

 The fact that certain degrees of obliteration of the axial gradi- 

 ents by differential inhibition produce an approach to more 

 primitive larval types, is particularly interesting as suggesting 

 that the evolution of the pluteus has consisted to some extent 

 in an evolution of the axial gradients, or, more correctly speak- 

 ing, of the protoplasmic conditions which determine the estab- 

 lishment, metabolic slope and physiological effectiveness of such 

 gradients. The pluteus, in short, shows a higher degree of 

 axiation than the crinoid larva, i.e., the axes appear more dis- 

 tinctly in form and differentiation of the body, and there is 



