62 Edmund B. JVilson. 



later movements and progressive segregation of the cytoplasmic 

 materials. I will only call attention, among these, first, to the 

 determination of the apical organ by material originally lying In 

 the lower polar area, which, if my interpretation of the experi- 

 ments is valid, moves upwards to the apical pole in the period 

 between the first and second cleavages. That such a movement 

 occurs is only a matter of inference ; but this interpretation appears 

 to me far simpler and more intelligible than to assume a brief 

 "Fernwirkung," or the like emanating from the first but not the 

 second polar lobe. It Is however not a matter of inference but 

 of fact that the remaining material of the lower white area moves 

 upwards and towards one side in the 8-cell stage preceding the 

 fourth cleavage, when it apparently fuses with the material of the 

 upper white area in the D-quadrant. It Is interesting to com- 

 pare this with the facts described by Vejdovsky in Rhynchelmis, 

 where the remains of the upper and lower polar rings fuse in the 

 D-quadrant at the 4-cell stage. 



I have endeavored to show that cytoplasmic prelocalizatlon 

 in Dentalium differs only In degree from the conditions existing in 

 such eggs as those of the nemertine or sea-urchin. The same may 

 be said, I think, of the development of isolated blastomeres, de- 

 spite the fact that in Dentalium such blastomeres are incapable 

 of producing complete dwarf embryos. As In the nemertine or 

 sea-urchin, although the isolated blastomere segments as a part 

 and not as a whole, the embryo finally closes, in the course of 

 which process structures like the prototroch, the post-trochal and 

 pre-trochal regions, and the gut, close to form whole structures. 

 That this process, which in the case of the nemertine I compared 

 to Morgan's "morphallaxis" In regenerating planarians or hy- 

 drolds, falls short of producing a complete embryo in Dentalium, 

 may be due to different causes in different cells. In the AB half 

 or one of the smaller quarters this Is obviously due in the main 

 to lack of the specific material of the lower polar area. The 

 failure of the CD half or the D quarter may In part be due to 

 a like cause; but since these embryos contain the materials (those 

 contained in the lower polar area) that are missing In the other 

 cases, their failure may be due to a different cause. The CD half 



