Experimental Studies on Germinal Localization. 57 



with the polar lobe or lower polar area is wholly disproportionate 

 to the effect produced. The polar lobe includes less than one- 

 fifth the volume of the egg; yet its removal does not merely cause 

 a structural defect of like extent, but inhibits the whole process 

 of growth and differentiation in the post-trochal region and the 

 concomitant withdrawal of the pre-trochal region. The cleavage 

 of the lobeless embryos shows that both the second and the third 

 quartets are formed; and it is fair to conclude that certainly in 

 the AB half of the embryo, and probably also in the CD half, 

 these cells contain ectoblastic material, which in a normal embryo 

 would contribute to the formation of the post-trochal region. 

 These cells, as stated above, close in around the posterior region, 

 and perhaps are partially turned in with the invaginating ento- 

 blast-cells. In any case, however, the power of active growth in 

 the post-trochal region, so conspicuous in the normal larva, is 

 wholly lost with the removal of the excess of material in the D 

 quadrant. It does not seem possible that this loss in power of 

 growth is due to mechanical obstacles, since the same defects exist 

 in fragments of the unsegmented egg from which the lower 

 polar area has been removed and which are free to segment as 

 best they can. The conclusion therefore appears unavoidable that 

 the material of the lobe is not only specifically necessary for the 

 formation of the bases of the post-trochal structures, but also for 

 the whole growth-process that is here brought to a focus. Apart 

 from its more general bearings, this conclusion is important from 

 the light that it may throw on the teloblastic growth of annelids 

 and other segmented forms, and it seems altogether probable 

 that if the polar lobe could be removed from such an egg as that 

 of Sabellaria or Myzostoma the resulting larva would fail to 

 develop a metameric trunk-region. 



A second point of interest that clearly appears from the ex- 

 periments is that the topographical grouping of specific materials 

 in the unsegmented egg may be in its ensemble widely different 

 from that of the definitive bases of the organs which they de- 

 termine; for the experiments demonstrate that the apical organ, 

 lying at the upper pole, is determined by material originally lying 

 far down in the vegetative hemisphere in the lower polar area. 



