NOTES ON REGULATION IN STYLARIA LACUSTRIS. 189 



FIG. 13. Oblique growth of bud of prostomium in (/>), and slightly incomplete 

 regeneration. Stages (b, c, d) were 4, 8 and 18 days after the operation, respec- 

 tively. The eye was not regenerated. The other eye increased in size. 



FIG. 14. Failure of regeneration in a sexual individual after removal of two seg- 

 ments. This condition existed nineteen days after cut was made. 



FIG. 15. Dwarfed anterior regeneration from a sexual individual after removal of 

 fifteen segments. The prostomium is not regenerated. A pharyngeal region of five 

 short segments is produced, as shown by the number of setae bundles. In (c) a side 

 view is given. The level at which the cut was made is shown in (a). 



FIG. 1 6. Regeneration from posterior end of a sexual individual. The part pro- 

 duced is of less diameter. 



enough to determine whether the regeneration of the sexual or- 

 gans was complete. The cut was made just behind the clitellum. 



CONCLUSIONS. 



1. The formation of a regenerating region will under certain 

 conditions inhibit the process of asexual multiplication and cause 

 the disappearance of the zone of fission. This effect may be pro- 

 duced by a cut anterior to the zone of fission, less often by a cut 

 posterior to it, and occurs only when the zone is embryonic. 

 The zone is also more likely to disappear if the cut is near to it. 

 The band of transparent embryonic tissue redifferentiates and the 

 energy of growth is transferred to the regenerating region. 



2. There is evidence of disorganization and a return to embry- 

 onic conditions in organs just behind a cut surface. This effect 

 does not appear to be a direct mechanical result, /. c., due to 

 crushing. Fragmentation of the pigmented portion of the eye is 

 one case adduced. 



3. Internal conditions favorable to proliferation, such as the 

 exposure of cut surfaces of intestine and blood-vessels, are pres- 

 ent in nearly all possible experiments. But if a corner of the 

 head segment be removed, including the prostomium, without 

 injuring the pharynx, the ectoderm may close over the surface 

 and regeneration may fail to take place. 



4. Short posterior pieces often fail to regenerate anteriorly, 

 but no cases of heteromorphosis, such as individuals with tails on 

 the anterior end, have been obtained. Short posterior pieces 

 which failed to regenerate within the time of observation may 

 elongate and show some differentiation in the budding region. 



5. The middle portion of the body has the greatest power of 

 regeneration, the specialized pharyngeal region has the least. 



