68 Edmund B. PFilson. 



mass. This sufficiently Indicates the complexity of the problem 

 with which we are dealing, and the importance of further more 

 precise studies of the facts. At the same time, it seems clear that 

 the problem of proportionate development in a fragment of an 

 organism here appears in a much simpler form than In a blastula- 

 fragment, or a piece of an adult organism such as a planarlan or 

 a hydra ; and I think we should not abandon the hope of finding 

 for It a relatively simple solution. While I am not able to offer 

 such a solution, It seems to me that it would be rash to deny its 

 possibility, not merely in the present instance, but In all analogous 

 processes, even when they take place under the more complex 

 conditions existing In multicellular masses. 



VIII. 



SUMMARY. 



1. The Dentalium egg shows from the beginning three hori- 

 zontal zones, an equatorial pigment-zone and two white polar 

 areas. Each of the polar areas includes a specially modified pro- 

 toplasmic area probably comparable to a polar ring. 



2. During cleavage the pigmented zone Is allotted mainly to 

 the entomeres, the upper white area to the ectomeres, the lower 

 white area to the first and probably also the second somatoblast. 

 At the first, second and third cleavages the lower white area tem- 

 porarily passes into the "yolk-lobe" or polar lobe. 



3. Removal of the first polar lobe leads to a symmetrical 

 cleavage without the subsequent formation of polar lobes, and to 

 the formation of a larva devoid of post-trochal region and apical 

 organ. Removal of a portion of the first lobe produces a larva 

 with reduced post-trochal reglon,^ and with or without apical organ. 

 Removal of the second polar lobe produces a larva without post- 

 trochal region but with an apical organ. 



4. The lobeless larvae undergo no metamorphosis, form no 

 foot, shell-gland or shell, no mantle-folds, no pedal ganglia, ap- 

 parently no mouth, and probably no coelomesoblast-bands. 



5. The isolated AB half or A, B, or C quarter, produces a 

 closed larva closely similar except In size, to the lobeless ones. 

 The isolated CD half or D quarter produces a larva possessing a 



