Experimental Studies on Germinal Localization. 69 



post-trochal region as large as In a normal larva, and an apical 

 organ, which dies without undergoing metamorphosis. The CD 

 half from which the second polar lobe is removed produces a larva 

 like that from an AB half, but possesses an apical organ. 



6. The isolated micromere id produces a mass of ectoblast- 

 cells bearing an apical organ, while la, ib, ic produce no apical 

 organ, 



7. Fertilized fragments of the unsegmented unfertilized egg, 

 obtained by horizontal or oblique section, differ in development 

 according as they do or do not contain the lower white area. 

 The upper fragment segments symmetrically without the forma- 

 tion of polar lobes and produces a larva similar to the lobeless 

 ones. The lower one segments like a whole egg of diminished 

 size, and may produce a normally formed dwarf trochophore. 



8. Fragments obtained by vertical section through the lower 

 white area may segment like whole eggs and may produce nearly 

 normally formed dwarf trochophores. 



9. Enucleated fragments, containing the lower white area, 

 of fertilized eggs, pass through alternating periods of activity 

 and quiescence corresponding with the division-rhythm of the 

 nucleated half, and form the polar lobes as if still forming part 

 of a complete embryo. The same is true of the isolated polar 

 lobe. 



10. The foregoing observations demonstrate the prelocallza- 

 tlon of specific cytoplasmic stuffs In the unsegmented egg and 

 their isolation in the early blastomeres. The lower white area 

 contains such stuffs that are essential to the formation of the 

 apical organ and the complex of structures forming the post- 

 trochal region, including the shell-gland and shell, the foot, the 

 mantle-folds and probably the coelomesoblast. These stuffs are 

 contained in the first polar lobe, but the second lobe no longer 

 contains those necessary for the basis of the apical organ. Pro- 

 gressive changes therefore occur in the original distribution of 

 the specific cytoplasmic materials. 



11. Comparison Indicates that the conditions observed In the 

 molluscan egg differ only In degree from those In the nemertlne 

 or echlnoderm. These differences reduce themselves to differ- 



