68 C. M. CHILD 



should be possible, not only to produce differential or graded 

 inhibition or acceleration of development along an axis, but to 

 determine the position of the greatest degree of inhibition or 

 acceleration, either at the high or low end, the region of highest 

 or that of lowest rate in the axis. To obtain a particular de- 

 gree of differential inhibition, the procedure must of course be 

 varied somewhat according to the toxicity of the reagent. 



The experiments on the developmental stages of the sea urchin 

 were undertaken with these ideas in mind and with the purpose 

 of demonstrating that the axial gradients are determining and 

 effective factors in embryonic development. The results of the 

 experiments have completely realized expectation and leave no 

 doubt as to the fundamental character of the role played by the 

 axial gradients in the development of the sea urchin. Some simi- 

 lar data on the starfish, which I hope to supplement by further 

 experiments before publication, indicate that the relations are 

 essentially the same there as in the sea urchin. 



THE AXIAL RELATIONS IN NORMAL DEVELOPMENT 



Throughout the paper the terms 'apical' and 'basal' are used 

 to designate the two ends of the apico-basal, polar, or major 

 axis, the apical end representing the apical or 'animal,' the 

 basal end the 'vegetative' pole of the egg. In speaking of 

 metabolic gradients, the ends or levels of different metabolic 

 rate in a gradient are distinguished as high and low, or higher 

 and lower ends or levels. 



According to Boveri ('01 a ,'01 b) the micromeres arise at the 

 basal pole, and the micromere region gives rise to the mesenchyme, 

 but Garbowski ('05) has shown that polarity of cleavage and of 

 the later stages does not always correspond to the axis indi- 

 cated by the pigment ring in Strongylocentrotus lividus. The 

 elongation of the blastula occurs in the direction of the apico- 

 basal axis, and the apical pole of the blastula represents the 

 apical pole of the egg. Before gastrulation, the basal region of 

 the blastula wall becomes thicker than the apical (fig. 1). In 

 the gastrula (fig. 2) the apex of the conical body represents the 

 apical end of the major axis, and the blastopore region the basal 

 end, while the invaginated entoderm represents a still more 



