30 Edmund B. Wilson. 



Since the CD >4 larva is less than ^ and the D }i larva less 

 than ^ the volume of the lobeless embryo, yet both produce 

 apical organ and post-trochal region, the conclusion is unavoid- 

 able that the failure to form these structures after removal of 

 the polar lobe must be due to a qualitative and not a quantitative 

 difference; in other words, the material of the lobe must be spe- 

 cifically different from the remaining material, and as such is 

 the determining cause of the development of the structures in 

 question. 



The above conclusion is fully sustained by the effect of cutting 

 off only a part of the polar lobe. In such embryos during the 

 second and third cleavages the polar lobe is correspondingly 

 diminished in size (Figs. 27, 28), and the D-quadrant is too 

 small by the same amount. Such eggs produce larvae with a cor- 

 responding reduction in the post-trochal region (Fig. 35) and 

 these larvae sometimes possess, sometimes lack, the apical organ. 

 It is not improbable therefore that further experiments of this 

 kind may show a localization, within the polar lobe itself, of the 

 determining materials of the apical organ and of the post-trochal 

 region. This experiment adds to the foregoing the important 

 result that after the polar lobe has formed there is a direct quan- 

 titative relation between the amount of specific material it con- 

 tains and the size of the post-trochal region, there being appar- 

 ently no regulative process in the later stages (though I have not 

 yet sufficiently examined this latter point). As will appear in 

 Part V, this conclusion does not apply to the material of the lower 

 polar area before the formation of the lobe. 



(b) The mesoblast question. — We may now consider what 

 is in some respects the most interesting, as it is certainly 

 the most difficult, of the questions relating to the lobeless 

 larvae, namely, that of the mesoblast. The fact that cer- 

 tainly the first and probably the second somatoblast is de- 

 rived mainly from the substance of the polar lobe, and 

 that after the removal of this substance the post-trochal region 

 fails to develop, suggests that the material of the coelomesoblast 

 as well as of the ectoblastic structures, is localized in the polar 

 lobe and hence in the original polar area. In point of fact 



